NAPLEX Quiz: Drug Concentrations And Ratio Strengths

20 questions · exam conditions

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Drug Concentrations And Ratio StrengthsQuestion 1 of 20

How many mL of a 1:2000 w/v solution provide 15 mg of drug?

30 mL

7.5 mL

3 mL

300 mL

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NAPLEX Quiz

NAPLEX Quiz: Drug Concentrations And Ratio Strengths

Practice Drug Concentrations And Ratio Strengths in NAPLEX with focused quiz questions that help you check what you know, review explanations, and build confidence with test-style prompts.

What this quiz covers

This quiz focuses on Drug Concentrations And Ratio Strengths, giving you a quick way to practice the rules, question types, and explanations that matter most for NAPLEX.

How to use this quiz

Try each quiz question before looking at the correct answer. Use the explanations to review missed ideas, then come back to similar questions until the pattern feels familiar.

All questions

Question 1

How many mL of a 1:2000 w/v solution provide 15 mg of drug?

30 mL (correct answer)
7.5 mL
3 mL
300 mL

Explanation: A 1:2000 w/v solution contains 1 g per 2000 mL, which is 1000 mg in 2000 mL, or 0.5 mg/mL. Divide 15 mg by 0.5 mg/mL to get 30 mL. A tempting error is 7.5 mL from treating the concentration as 2 mg/mL, but that inverts the ratio and is incorrect.

Question 2

How many mL of 1:1000 w/v stock are required to prepare 500 mL of 1:10,000 w/v?

5 mL
50 mL (correct answer)
500 mL
0.5 mL

Explanation: A 1:10,000 solution is 10 times more dilute than 1:1000, so you need one tenth of the final volume as stock: 500 mL divided by 10 equals 50 mL. The tempting wrong answer is 5 mL, which treats this as a 100-fold dilution instead of the actual 10-fold dilution.

Question 3

What ratio strength is equivalent to 0.025% w/v?

1:40
1:400
1:40000
1:4000 (correct answer)

Explanation: 0.025% w/v means 0.025 g per 100 mL. Scaling to 1 g gives 100 / 0.025 = 4,000 mL, so the strength is 1:4000. The tempting wrong choice is 1:40000, which comes from moving the decimal one place too far and treating 0.025% as 1 in 40,000 instead of 1 in 4,000.

Question 4

Which concentration contains the most drug per milliliter?

2.5 mg/mL
0.3% w/v
1:250 w/v (correct answer)
1:1000 w/v

Explanation: Convert all to mg/mL: 2.5 mg/mL stays 2.5; 0.3% w/v is 3 mg/mL; 1:250 w/v is 1000 mg / 250 mL = 4 mg/mL; 1:1000 w/v is 1 mg/mL. The 2.5 mg/mL choice is tempting because it is already per mL, but it falls below the converted 3 and 4 mg/mL values.

Question 5

A 1:4000 w/v solution contains how many milligrams per 25 mL?

6.25 mg (correct answer)
0.625 mg
62.5 mg
625 mg

Explanation: A 1:4000 w/v solution contains 1 gram, or 1000 mg, in 4000 mL. Since 4000 mL divided by 25 mL is 160, you divide 1000 mg by 160 and get 6.25 mg. The tempting error is 0.625 mg, which would be 1:40,000 instead of 1:4000.

Question 6

A 52-year-old male (weight 90 kg) presents with an acute gout flare and requests a compounded topical NSAID for localized pain. Allergies: aspirin (bronchospasm). Current medications: allopurinol 300 mg PO daily, amlodipine 10 mg PO daily. Labs: SCr 1.0 mg/dL, AST 20 U/L, ALT 17 U/L, K 4.0 mEq/L. The prescriber requests diclofenac in a compounded gel at a ratio strength of 1:50 (w/w), total quantity 100 g. Calculate the ratio strength for this compounded medication in terms of grams of diclofenac required for the final product.

1 g diclofenac in 100 g total
5 g diclofenac in 100 g total
2 g diclofenac in 100 g total (correct answer)
0.5 g diclofenac in 100 g total

Explanation: This question tests understanding of ratio strength calculations in pharmaceutical compounding, specifically for topical preparations. The key principle is that a 1:50 ratio strength (w/w) means 1 part active ingredient to 50 parts total product, which equals 2% concentration. The correct answer is C (2 g diclofenac in 100 g total) because 1:50 means 1 g drug per 50 g total product, so for 100 g total, we need 2 g of diclofenac (100 g ÷ 50 = 2 g). Answer A (1 g) would create a 1:100 ratio, too dilute for the prescription. Answer B (5 g) would yield a 1:20 ratio, too concentrated. Answer D (0.5 g) would produce a 1:200 ratio, far below the prescribed strength. When working with ratio strengths, remember that the second number represents total parts, not just the base, and always convert between ratio strength and percentage to verify accuracy. For topical NSAIDs, maintaining the correct concentration ensures therapeutic efficacy while minimizing systemic absorption and adverse effects.

Question 7

A 70-year-old man (weight 80 kg) is started on heparin for treatment of pulmonary embolism (high-alert medication). Allergies: none. Current medications: none. Labs: SCr 1.0 mg/dL, AST/ALT 26/24 units/L, platelets 210,000/mm $^3$ , Na 138 mEq/L. The nurse requests a heparin infusion bag prepared as 25,000 units in 500 mL of 0.9% sodium chloride. What is the final concentration of the IV preparation?

$25\,\text{units/mL}$
$50\,\text{units/mL}$ (correct answer)
$100\,\text{units/mL}$
$500\,\text{units/mL}$

Explanation: The pharmaceutical calculation concept being tested is calculating the concentration of an anticoagulant IV infusion. The key mathematical principle is dividing total units by total volume to find units/mL. The correct answer 50 units/mL is best because 25,000 units / 500 mL = 50 units/mL, standard for heparin protocols. Choice A 25 units/mL is incorrect, perhaps from halving the units. Choices C 100 units/mL and D 500 units/mL are wrong, from volume errors like using 250 mL or 50 mL. A transferable clinical pearl is to monitor aPTT regularly during heparin therapy. Emphasizing high-alert medication calculations reduces thrombosis risks.

Question 8

A 61-year-old woman (weight 58 kg) is admitted for severe nausea and vomiting. Allergies: ondansetron (headache). Current medications: sertraline 50 mg daily. Labs: SCr 0.7 mg/dL, AST/ALT 20/19 units/L, Na 136 mEq/L, K 3.8 mEq/L. The provider orders metoclopramide 10 mg IV to be prepared in a syringe at a concentration of $2\,\text{mg/mL}$ ; metoclopramide injection is available as $5\,\text{mg/mL}$ . Calculate the amount of drug required for this concentration (mL of metoclopramide injection needed) to prepare 5 mL final volume.

1 mL
2 mL (correct answer)
4 mL
5 mL

Explanation: The pharmaceutical calculation concept being tested is calculating the volume of stock solution needed for a diluted IV preparation at a specific concentration and final volume. The key mathematical principle is determining drug amount from final concentration and volume, then dividing by stock concentration. The correct answer 2 mL is best because 10 mg total (from 2 mg/mL × 5 mL) divided by 5 mg/mL stock equals 2 mL needed. Choice A 1 mL is incorrect, halving the required volume and yielding only 5 mg total. Choices C 4 mL and D 5 mL are wrong, providing excess drug like 20 mg or 25 mg, possibly from confusing final and stock concentrations. A transferable clinical pearl is to prepare small-volume IV pushes with precise dilutions to minimize waste. Emphasizing calculation of total dose first ensures safe antiemetic administration.

Question 9

A 4-year-old girl (weight 16 kg) is diagnosed with streptococcal pharyngitis. Allergies: none. Current medications: none. Labs: SCr 0.3 mg/dL, AST/ALT 19/17 units/L, Na 140 mEq/L, K 4.0 mEq/L. The prescriber orders azithromycin suspension and the pharmacy has a bottle labeled $200\,\text{mg}/5\,\text{mL}$ . What is the concentration of the prepared solution in $\text{mg/mL}$ ?

$20\,\text{mg/mL}$
$40\,\text{mg/mL}$ (correct answer)
$50\,\text{mg/mL}$
$200\,\text{mg/mL}$

Explanation: The pharmaceutical calculation concept being tested is converting a labeled suspension concentration to mg/mL units. The key mathematical principle is dividing the drug amount by the corresponding volume from the label. The correct answer 40 mg/mL is best because 200 mg divided by 5 mL equals 40 mg/mL, providing the per-milliliter strength. Choice A 20 mg/mL is incorrect, possibly from dividing by 10 mL. Choices C 50 mg/mL and D 200 mg/mL are suboptimal, likely from misinterpreting the label or ignoring the volume. A transferable clinical pearl is to shake suspensions well before measuring to ensure uniform concentration. Emphasizing unit conversions aids accurate pediatric antibiotic dosing.

Question 10

A 63-year-old man (weight 74 kg) is in the ICU with septic shock requiring norepinephrine. Allergies: none. Current medications: none. Labs: SCr 1.6 mg/dL, AST/ALT 45/40 units/L, Na 134 mEq/L, K 4.8 mEq/L. The order is to prepare norepinephrine 8 mg in 250 mL of 5% dextrose in water. What is the final concentration of the IV preparation?

$8\,\text{mcg/mL}$
$16\,\text{mcg/mL}$
$32\,\text{mcg/mL}$ (correct answer)
$64\,\text{mcg/mL}$

Explanation: The pharmaceutical calculation concept being tested is calculating the concentration of a vasopressor IV infusion, including unit conversion to mcg/mL. The key mathematical principle is dividing total drug by volume, then converting mg to mcg for clinical relevance. The correct answer 32 mcg/mL is best because 8 mg / 250 mL = 0.032 mg/mL, and 0.032 × 1000 = 32 mcg/mL. Choice A 8 mcg/mL is incorrect, quartering the value perhaps by misdividing. Choices B 16 mcg/mL and D 64 mcg/mL are wrong, from halving or doubling errors in conversion. A transferable clinical pearl is to standardize vasopressor concentrations in ICUs for rapid titration. Emphasizing mcg/mL units prevents overdose in shock management.

Question 11

A 9-year-old girl (weight 28 kg) is prescribed diphenhydramine for urticaria. Allergies: none. Current medications: none. Labs: SCr 0.5 mg/dL, AST/ALT 17/15 units/L, Na 139 mEq/L, K 4.2 mEq/L. The pharmacy stocks diphenhydramine liquid labeled $12.5\,\text{mg}/5\,\text{mL}$ . What is the concentration of the prepared solution in $\text{mg/mL}$ ?

$0.25\,\text{mg/mL}$
$1.25\,\text{mg/mL}$
$2.5\,\text{mg/mL}$ (correct answer)
$12.5\,\text{mg/mL}$

Explanation: This question tests the concept of drug concentration conversion from a labeled ratio to milligrams per milliliter (mg/mL). The key mathematical principle involves dividing the amount of drug by the volume to express the concentration in standardized units, independent of patient-specific factors like weight or labs, which are provided as distractors. The correct answer, 2.5 mg/mL, is the best calculation because dividing 12.5 mg by 5 mL yields exactly 2.5 mg/mL, accurately reflecting the stock solution's concentration without any preparation or dilution implied. Choice A (0.25 mg/mL) is incorrect due to a common error of dividing by 50 instead of 5, perhaps misreading the label; choice B (1.25 mg/mL) might result from halving the correct value or confusing with a different formulation; choice D (12.5 mg/mL) erroneously assumes the label indicates per mL rather than per 5 mL. Always verify units when converting concentrations to avoid dosing errors in pediatric patients. A useful strategy is to double-check calculations by multiplying back: 2.5 mg/mL times 5 mL equals 12.5 mg, confirming accuracy.

Question 12

A 6-year-old boy (weight 20 kg) is seen for acute otitis media. Allergies: none. Current medications: none. Labs: SCr 0.4 mg/dL, AST/ALT 20/18 units/L, Na 139 mEq/L, K 4.1 mEq/L. The prescriber orders amoxicillin $45\,\text{mg/kg/day}$ by mouth divided every 12 hours for 10 days; the pharmacy stocks amoxicillin suspension $400\,\text{mg}/5\,\text{mL}$ . What is the concentration of the prepared solution in $\text{mg/mL}$ ?

$40\,\text{mg/mL}$
$80\,\text{mg/mL}$ (correct answer)
$100\,\text{mg/mL}$
$400\,\text{mg/mL}$

Explanation: The pharmaceutical calculation concept being tested is converting a labeled concentration to a different unit for oral suspensions. The key mathematical principle is dividing the amount of drug by the volume to find mg/mL, independent of patient-specific dosing. The correct answer 80 mg/mL is best because 400 mg divided by 5 mL equals 80 mg/mL, accurately reflecting the stock concentration. Choice A 40 mg/mL is incorrect, halving the actual strength perhaps by misreading the label. Choices C 100 mg/mL and D 400 mg/mL are wrong, possibly from ignoring the volume or confusing with daily dose calculations. A transferable clinical pearl is to verify suspension concentrations before calculating pediatric doses to avoid under- or overdosing. Emphasizing unit conversions like mg per mL ensures safe dispensing and administration in children.

Question 13

A 52-year-old man (weight 84 kg) with COPD exacerbation needs IV methylprednisolone. Allergies: none. Current medications: tiotropium inhaler daily, albuterol inhaler as needed. Labs: SCr 1.0 mg/dL, AST/ALT 30/27 units/L, Na 142 mEq/L, K 4.4 mEq/L. The order is methylprednisolone sodium succinate 125 mg IV; you reconstitute a 125 mg vial with 2 mL diluent to yield a concentration of $62.5\,\text{mg/mL}$ . What is the concentration of the prepared solution?

$31.25\,\text{mg/mL}$
$62.5\,\text{mg/mL}$ (correct answer)
$125\,\text{mg/mL}$
$250\,\text{mg/mL}$

Explanation: The pharmaceutical calculation concept being tested is determining the concentration after reconstituting a steroid vial with a specified diluent volume. The key mathematical principle is dividing the drug amount by the added diluent volume, assuming negligible powder displacement for simplicity. The correct answer 62.5 mg/mL is best because 125 mg divided by 2 mL equals 62.5 mg/mL, as stated in the reconstitution process. Choice A 31.25 mg/mL is incorrect, perhaps from dividing by 4 mL in error. Choices C 125 mg/mL and D 250 mg/mL are suboptimal, likely from not accounting for dilution or misreading the vial strength. A transferable clinical pearl is to check package inserts for exact reconstitution volumes to account for powder volume. Emphasizing unit consistency in mg/mL prevents errors in corticosteroid dosing.

Question 14

A 62-year-old woman (weight 66 kg) with severe pain is ordered morphine IV. Allergies: codeine (nausea). Current medications: acetaminophen 650 mg every 6 hours as needed. Labs: SCr 0.8 mg/dL, AST/ALT 21/19 units/L, Na 139 mEq/L, K 4.2 mEq/L. The order is morphine 4 mg IV; morphine injection is available as $10\,\text{mg/mL}$ and must be diluted to a final concentration of $1\,\text{mg/mL}$ . Determine the appropriate dilution: what final volume (mL) should contain the 4 mg dose?

0.4 mL
4 mL (correct answer)
10 mL
40 mL

Explanation: The pharmaceutical calculation concept being tested is calculating the final volume for a diluted opioid injection to reach a target concentration. The key mathematical principle is dividing the dose by the desired concentration. The correct answer 4 mL is best because 4 mg / 1 mg/mL = 4 mL total volume containing the dose. Choice A 0.4 mL is incorrect, matching undiluted stock volume but not the concentration. Choices C 10 mL and D 40 mL are suboptimal, yielding 0.4 mg/mL or 0.1 mg/mL, excessively dilute. A transferable clinical pearl is to monitor respiration after opioid administration. Emphasizing dilution for safety reduces injection risks.

Question 15

A 27-year-old woman (weight 60 kg) is seen for acne. Allergies: none. Current medications: none. Labs: SCr 0.7 mg/dL, AST/ALT 17/15 units/L, Na 139 mEq/L, K 4.1 mEq/L. A dermatologist requests a compounded topical clindamycin solution labeled as 1% w/v. Calculate the ratio strength for this compounded medication.

1:10
1:50
1:100 (correct answer)
1:1,000

Explanation: The pharmaceutical calculation concept being tested is converting percentage strength to ratio strength for topical solutions. The key mathematical principle is that percent w/v equals grams per 100 mL, so ratio is 1 : (100 / percent). The correct answer 1:100 is best because for 1% w/v, 100 / 1 = 100, meaning 1 part clindamycin in 100 parts total solution. Choice A 1:10 is incorrect, representing 10% which is too concentrated for acne therapy. Choices B 1:50 and D 1:1,000 are wrong, corresponding to 2% and 0.1%, from arithmetic errors like halving or decimal shifts. A transferable clinical pearl is to use w/v for liquids in compounding to match solubility considerations. Emphasizing ratio verification enhances safety in dermatologic preparations.

Question 16

A 66-year-old woman (weight 62 kg) is admitted for acute decompensated heart failure and needs IV furosemide. Allergies: sulfonamides (rash). Current medications: carvedilol 12.5 mg twice daily, spironolactone 25 mg daily. Labs: SCr 1.2 mg/dL, AST/ALT 32/29 units/L, Na 132 mEq/L, K 5.1 mEq/L. The order requests furosemide 40 mg IV in a syringe diluted to a final concentration of $2\,\text{mg/mL}$ ; furosemide injection is $10\,\text{mg/mL}$ . Determine the appropriate dilution for this medication: what final volume (mL) should the syringe contain?

10 mL
20 mL (correct answer)
40 mL
50 mL

Explanation: The pharmaceutical calculation concept being tested is calculating the final volume for a diluted IV diuretic to achieve a target concentration. The key mathematical principle is dividing the dose by the desired concentration to find total volume. The correct answer 20 mL is best because 40 mg / 2 mg/mL = 20 mL, ensuring safe push administration. Choice A 10 mL is incorrect, yielding 4 mg/mL which is too concentrated. Choices C 40 mL and D 50 mL are wrong, resulting in 1 mg/mL or 0.8 mg/mL, unnecessarily dilute. A transferable clinical pearl is to monitor electrolytes post-diuretic administration. Emphasizing stock concentration in dilutions avoids vein irritation.

Question 17

A 40-year-old woman (weight 72 kg) presents with allergic rhinitis and requests a compounded nasal solution. Allergies: none. Current medications: cetirizine 10 mg daily. Labs: SCr 0.8 mg/dL, AST/ALT 16/14 units/L, Na 140 mEq/L, K 4.2 mEq/L. The prescription is for phenylephrine nasal solution 0.25% w/v. Calculate the ratio strength for this compounded medication.

1:4
1:40
1:400 (correct answer)
1:4,000

Explanation: The pharmaceutical calculation concept being tested is converting low-percentage strength to ratio strength for nasal solutions. The key mathematical principle is ratio = 1 : (100 / percent) for w/v concentrations. The correct answer 1:400 is best because for 0.25% w/v, 100 / 0.25 = 400, indicating 1 part phenylephrine in 400 parts solution. Choice A 1:4 is incorrect, equating to 25% which is overly concentrated. Choices B 1:40 and D 1:4,000 are suboptimal, representing 2.5% and 0.025%, from decimal point errors. A transferable clinical pearl is to consider osmolarity in nasal compounds to avoid irritation. Emphasizing precise conversions ensures effective decongestant therapy.

Question 18

A 58-year-old man (weight 79 kg) is admitted with alcohol withdrawal and needs thiamine IV. Allergies: none. Current medications: none. Labs: SCr 0.8 mg/dL, AST/ALT 55/48 units/L, Na 135 mEq/L, K 3.9 mEq/L. The order is thiamine 100 mg IV added to 50 mL of 0.9% sodium chloride. What is the final concentration of the IV preparation?

$0.2\,\text{mg/mL}$
$1\,\text{mg/mL}$
$2\,\text{mg/mL}$ (correct answer)
$20\,\text{mg/mL}$

Explanation: The pharmaceutical calculation concept being tested is determining the final concentration of a vitamin IV admixture. The key mathematical principle is dividing the drug amount by the total volume. The correct answer 2 mg/mL is best because 100 mg / 50 mL = 2 mg/mL, appropriate for withdrawal management. Choice A 0.2 mg/mL is incorrect, perhaps from using 500 mL volume. Choices B 1 mg/mL and D 20 mg/mL are suboptimal, from doubling volume or not dividing. A transferable clinical pearl is to administer thiamine before glucose in alcoholics. Emphasizing mg/mL units ensures proper IV preparation.

Question 19

A 54-year-old man (weight 86 kg) is undergoing procedural sedation. Allergies: none. Current medications: amlodipine 10 mg daily. Labs: SCr 0.9 mg/dL, AST/ALT 25/22 units/L, Na 140 mEq/L, K 4.2 mEq/L. The order is to prepare midazolam 2 mg in a total volume of 10 mL for slow IV push; midazolam injection is supplied as $1\,\text{mg/mL}$ . Calculate the amount of drug required for this concentration (mL of midazolam injection needed) to prepare the syringe.

0.2 mL
2 mL (correct answer)
5 mL
10 mL

Explanation: The pharmaceutical calculation concept being tested is determining the stock volume needed for a diluted sedative IV push given total dose and final volume. The key mathematical principle is dividing the required dose by the stock concentration. The correct answer 2 mL is best because 2 mg / 1 mg/mL = 2 mL of stock, with diluent to reach 10 mL total. Choice A 0.2 mL is incorrect, providing only 0.2 mg which is subtherapeutic. Choices C 5 mL and D 10 mL are wrong, exceeding the dose with 5 mg or 10 mg. A transferable clinical pearl is to administer sedatives slowly to monitor respiratory effects. Emphasizing dose-first calculations prevents procedural errors.

Question 20

A 59-year-old woman (weight 70 kg) is admitted for atrial fibrillation with rapid ventricular response. Allergies: none. Current medications: metoprolol succinate 50 mg daily, apixaban 5 mg twice daily. Labs: SCr 0.9 mg/dL, AST/ALT 28/30 units/L, Mg 1.9 mg/dL, K 4.3 mEq/L. An order is written for diltiazem infusion: prepare a bag containing 125 mg diltiazem in a final volume of 250 mL of 0.9% sodium chloride. What is the final concentration of the IV preparation?

$0.25\,\text{mg/mL}$
$0.5\,\text{mg/mL}$ (correct answer)
$1\,\text{mg/mL}$
$2\,\text{mg/mL}$

Explanation: The pharmaceutical calculation concept being tested is calculating the final concentration of an IV admixture. The key mathematical principle is dividing the total drug amount by the total volume to obtain mg/mL. The correct answer 0.5 mg/mL is best because 125 mg diltiazem in 250 mL yields 125 / 250 = 0.5 mg/mL, suitable for infusion. Choice A 0.25 mg/mL is incorrect, perhaps from doubling the volume in error. Choices C 1 mg/mL and D 2 mg/mL are suboptimal, likely from halving the volume or miscalculating the numerator. A transferable clinical pearl is to label IV bags with exact concentrations to guide infusion rate adjustments. Emphasizing accuracy in unit consistency prevents dosing errors in critical care settings.