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AP Chemistry : Mass

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Example Questions

Example Question #1 : Compounds And Molecules

What is the percent by mass of bismuth in the compound $Bi_{2}Te_{3}}$ ?

Possible Answers:

$13.22\%$

$5.22\%$

$26.1\%$

$52.2\%$

$47.8\%$

Correct answer:

$52.2\%$

Explanation:

The mass percentage of bismuth in the compound will be equal to the mass of bismuth in one mole of compound divided by the total molar mass of the compound.

Bismuth has a molar mass of $208.98\frac{g}{mol}$ . One mole of the compound would result in two moles of bismuth, a total of 417.96g.

2(208.98)=417.96g

Tellurium has a molar mass of $127.6\frac{g}{mol}$ . One mole of the compound would result in three moles of tellurium, a total of 382.8g.

3(127.6)=382.8g

Add the mass of bismuth and the mass of tellurium per mole to find the total molar mass.

MM=417.96g+382.8g=800.76g

Divide the mass of bismuth by the total molecular mass to find the percent by mass of bismuth in the compound.

$\frac{417.96g}{800.76g}\times100\%=52.2\%$

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Example Question #1 : Ap Chemistry

What is the percent by mass of carbon in acetic acid $(CH_{3}COOH)$ ?

Possible Answers:

$53.29\%$

$6.71\%$

$36.1\%$

$40\%$

$19.89\%$

Correct answer:

$40\%$

Explanation:

The total molar mass of acetic acid is $60\frac{g}{mol}$ .

$2(12)+2(16)+4(1)=60\frac{g}{mol}$

Carbon contributes 24g total. Therefore, in a sample of 1 mole of acetic acid, there are 24g of carbon. To find the percent by mass, we must divide the mass of carbon by the total molar mass of the compound.

$\frac{24g}{60g}=40\%$

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Example Question #2 : Mass

Calculate the percent by mass of each element in $PbSO_{4}}$ .

Possible Answers:

$Pb: 68.33\%; S: 21.1\%; O: 10.57\%$

$Pb: 10.57\%; S: 21.1\%; O: 68.33\%$

$Pb: 21.1\%; S: 68.33\%; O: 10.57\%$

$Pb: 21.1\%; S: 10.57\%; O: 68.33\%$

$Pb: 68.33\%; S: 10.57\%; O: 21.2\%$

Correct answer:

$Pb: 68.33\%; S: 10.57\%; O: 21.2\%$

Explanation:

The total molar mass of lead (II) sulfate is 303.265g/mol . Lead contributes 207.2g , sulfur contributes 32.065g , and oxygen contributes (4*16g)=64g .

The percent by mass of each element in the compound is found by dividing the mass contribution of that element by the total molar mass of the compound.

$Pb: \frac{207.2gPb}{303.265gtotal}=68.33\%$

$S: \frac{32.065gS}{303.265gtotal}=10.57\%$

$O: \frac{64gO}{303.265gtotal}=21.1\%$

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Example Question #1 : Ap Chemistry

What percent by mass of $CaCl_{2}}$ is chlorine?

Possible Answers:

$63.9\%$

$36.1\%$

$0.361\%$

$53.06\%$

$46.9\%$

Correct answer:

$63.9\%$

Explanation:

The molar mass of calcium chloride is $110.987\frac{g}{mol}$ . Chlorine has two moles per one mole of $CaCl_{2}}$ . Therefore, chloride contributes 70.9g to the total molecular mass.

$(2mol*35.45\frac{g}{mol})=70.9g$

To find the percent by mass of chlorine in calcium chloride, divide the contribution of chlorine by the total molar mass.

$\frac{70.9g}{110.987g}=63.9\%$

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Example Question #3 : Compounds

Calculate the percent by mass of each element in $Al_{2}(CrO_{4})_{3}$ .

Possible Answers:

$Al: 16.76\%; Cr: 48.45\%; O: 34.79\%$

$Al: 31.75\%; Cr: 30.6\%; O: 37.65\%$

$Al: 13.42\%; Cr: 47.7\%; O: 38.8\%$

$Al: 13.42\%; Cr: 38.8\%; O: 47.7\%$

$Al: 28.4\%; Cr: 54.7\%; O: 16.9\%$

Correct answer:

$Al: 13.42\%; Cr: 38.8\%; O: 47.7\%$

Explanation:

The total mass of one mole of aluminum (II) chromate is calculated by:

$2(26.98\frac{g}{mol} Al)+3(51.99\frac{g}{mol} Cr)+12(16\frac{g}{mol}O)=401.95\frac{g}{mol}Al_2(CrO_4)_3$

Aluminum, chromate, and oxygen contribute 53.96g, 155.97g, and 192g respectively. Therefore, we can divide each contribution by the total molecular mass to determine percentages by mass.

$Al: \frac{53.96gAl}{401.95gtotal}=13.42\%$

$Cr: \frac{155.97gCr}{401.95gtotal}=38.8\%$

$O: \frac{192gO}{401.95gtotal}=47.78\%$

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Example Question #1 : Compounds

What is the percent by mass of hydrogen in $U(C_{2}H_{3}O_{2})_{3}$ ?

Possible Answers:

$0.2428\%$

$1.017\%$

$82.64\%$

$1.736\%$

$2.18\%$

Correct answer:

$2.18\%$

Explanation:

The total mass of $U(C_{2}H_{3}O_{2})_{3}$ is $415.16\frac{g}{mol}$ and calculated by:

$(238.0289\frac{g}{mol}U)+6(12.01\frac{g}{mol}C)+9(1.008\frac{g}{mol}H)+6(16\frac{g}{mol}O)=415.16\frac{g}{mol}U(C_2H_3O_2)_3$

You can then divide hydrogen's contribution by the total molecular mass in order to find the percent mass.

$\frac{9.072g}{415.16g}=2.18\%$

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Example Question #7 : Mass

What percent of the overall mass is represented by the cation(s) in $Cs_{2}Se$ ?

Possible Answers:

$37.3\%$

$62.7\%$

$22.9\%$

$50.0\%$

$77.1\%$

Correct answer:

$77.1\%$

Explanation:

The total molar mass of $Cs_{2}Se$ is $344.78\frac{g}{mol}$ . Cesium has a +1 charge when the compound is dissolved in solution and is, thus, the cation.

$Ce_2Se\rightarrow2Ce^{+1}+Se^{-2}$

Cesium contributes 265.8g to the total molecular mass.

$(2mol*132.9\frac{g}{mol}Cs)=265.8gCs$

We can find the percent by mass of cesium by dividing the mass of cesium by the total molecular mass.

$\frac{265.8gCs}{344.78g\ total}=77.1\%$

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AP Chemistry : Mass

Study concepts, example questions & explanations for AP Chemistry

All AP Chemistry Resources

Example Questions

Example Question #1 : Compounds And Molecules

Example Question #1 : Ap Chemistry

Example Question #2 : Mass

Example Question #1 : Ap Chemistry

Example Question #3 : Compounds

Example Question #1 : Compounds

Example Question #7 : Mass

All AP Chemistry Resources

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