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Example Questions
Example Question #29 : Atomic Structure And Properties
A chemist has found that 2.0 moles of a mysterious compound contained 48g of carbon, 8g of hydrogen, and 64g of oxygen. What is the molecular formula of the compound?
Find the number of moles of each element.
If the chemist has 2.0 moles of the compound, 1 mole would contain half as much. So 1mol of carbon, 4mol of hydrogen, and 2mol of oxygen, which forms acetic acid 
Example Question #31 : Atomic Structure And Properties
How many atoms are there in 
This question is asking us to determine how many atoms there are in 
Example Question #1 : Mass Spectroscopy Of The Elements
A certain element X is comprised of isotopes A, B, and C. Isotope A has a mass of 50 amu and is 70% of naturally occurring X. Isotope B is 35 amu and is 25% of X. Isotope C has a mass of 70 amu and is 5% of X. What is the atomic weight of element X?
55 g/mol
47 g/mol
87 g/mol
40 g/mol
70 g/mol
47 g/mol
0.70 (50 amu) + 0.25 (35 amu) + 0.05 (70 amu) = 47 amu = 47 g/mol
Example Question #2 : Mass Spectroscopy Of The Elements
The average molecular weight for element X is 59.97 g/mol. There are two known isotopes of element X, one weighing 59 g/mol, and the other weighing 61 g/mol. What is the relative abundance of each?
63% X-59, 37% X-61
75% X-59, 25% X-61
50% X-59, 50% X-61
51.5% X-59, 48.5% X-61
48.5% X-59, 51.5% X-61
51.5% X-59, 48.5% X-61
In this simplistic case, we only have two isotopes, so we know the following:
Where "n" represents the relative abundance of X-59, and "(1-n)" is the relative abundance of X-61. We will algebraically solve for "n":
If "n" is equal to 0.515, that means the relative abundance of X-59 is 51.5%, and thus the relative abundance of X-61 is 48.5%.
Example Question #3 : Mass Spectroscopy Of The Elements
Arbitrary element, X, has two isotopes, 
There is not enough information to answer this question
There must be a third isotope in order for this to occur
They are both present in equal amounts
They are both present in equal amounts
A calculation would show that the relative abundance of each is 50%, you can see that 8.000 g/mol is exactly the midpoint between the masses of 7 and 9, so each has to be present in equal quantities.
Example Question #3 : Mass Spectroscopy Of The Elements
An element has two main isotopes. The first isotope has a molar mass of 125 grams, and the second isotope has a molar mass of 128 grams. A naturally occurring sample of the element has a molar mass of 126.8 grams.
What is the lighter isotope's percentage in the natural sample?
In order to find the percentage of the lighter isotope in the natural sample, we need to use an equation that compares the percentage of each isotope and the natural molar mass. Since both of the isotopes together will account for 100% of the natural sample, we can say that the lighter isotope's percentage is 
Use this equation to solve for 
In other words, we can conclude that the lighter isotope makes up 40% of the natural sample.
Example Question #1 : Mass Spectroscopy Of The Elements
Chemical 
The abundance of 
There is not enough information to answer this question
The abundance of 
The abundance of 
The abundance of 
The abundance of
The abundance of 
The abundance of 
Example Question #5 : Mass Spectroscopy Of The Elements
Many elements exist in nature as varying isotopes. Although one isotope commonly predominates due to stability, the other isotopes can affect the overall mass of one natural mole of the element.
Suppose a mystery element has two main isotopes: isotope X has a molar mass of 128 grams per mole and isotope Y has a molar mass of 132 grams per mole. One mole of this element, when found naturally, weighs 128.8 grams. What are the percentages of isotopes X and Y when found in nature?
60% X and 40% Y
20% X and 80% Y
80% X and 20% Y
50% X and 50% Y
80% X and 20% Y
This answer becomes a lot easier if we use some quick mental math. First, notice that if the two isotopes were equal in nature (50/50), than a mole would weigh 130 grams.
Since the number is less than that, we can conclude that isotope X is more dominant than isotope Y, lowering the average mass. Now, we can use math to find the exact percentage.
Start by realizing that isotope X will make up some percentage of the element in nature (we will call this percentage "A") while isotope Y will make up the rest of that percentage (1-A).
This means that 80% of the element is composed of isotope X, and the other 20% is composed of isotope Y. Going back to our mental math, we see that, just as we predicted, isotope X is present in a larger amount than isotope Y.
Example Question #4 : Mass Spectroscopy Of The Elements
Which of the following is not a subatomic particle?
Protons, neutrons, and electrons are all subatomic particles. Alpha particles consist of 2 protons and 2 neutrons bound together in a particle. It is usually emitted during fission reactions.
Example Question #1 : Mass Spectroscopy Of The Elements
Which of the following statements is true?
Electrons are larger than protons, which means they must have less charge than protons
Electrons are smaller than protons, which means they have less charge than protons
Electrons are close in proximity to the atomic nucleus
The nucleus of an atom is electrically neutral
Electrons are smaller in size than nucleons, but have equal charge
Electrons are smaller in size than nucleons, but have equal charge
Electrons are notably smaller than nucleons (protons and neutrons), however, they have an equal amount of charge to protons. This is why an atom with the same amount of protons as electrons is considered electrically neutral.
The nucleus itself is positively charged, as the electrons are located relatively far away from the atomic center.
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