AP Chemistry - Elemental Composition of Pure Substances

Which answer choice shows the correct chemical formula for barium sulfate?

Explanation

We should first remember the difference between sulfate, sulfite, and sulfide. Sulfate is $SO_4^{-2}$ , sulfite is $SO_3^{-2}$ ` and sulfide is $S^{-2}$ .

The only answer choices that could be right must have $SO_4^{-2}$ in them. We then need to see that barium usually has a charge of , as the periodic table shows us, and so we need a charge of to cancel that out. The answer is .

What is the percent of carbon by mass in acetone CH3C(O)CH3?

62%

50%

75%

10%

25%

Explanation

To calculate the percent mass of a particular element within a substance I like to use a formulaic approach.

First multiply the number of moles of the atom of interest (C) by its molar mass.

3moles* 12 g/mol= 36g

Then divide by the overall molar mass of the molecule, obtain this by adding the atomic weights of each atom present in the molecule:

36g/58g= 62%

What is the percent composition of potassium in K2Cr2O7?

26.5%

35.4%

50%

22%

Explanation

The molecular weight of K2Cr2O7 is 2(39 g/mol) + 2(52 g/mol) + 7(16 g/mol) = 294 g/mol.

Out of this total weight, the percent composition of K is: (2(39 g/mol)/294 g) = 26.5%

Which of the following does not have the empirical formula CH?

C6H6

Ethyne

C3H4

C100H100

Explanation

An empirical formula represents a molecule with the simplest ratio. C6H6, ethyene (C2H2) and C100H100 can all be reduced to CH, however, C3H4 cannot because it does not have C and H atoms in a 1 : 1 ratio.

Which of the following compounds has a percent composition of carbon, (approximately 63%)?

Acetone

Butanol

Glucose

C3H8

Explanation

Acetone, which as a formula of C3H6O has a total molecular weight of: 3(12) + 6(1) + 1(16) = 58g/mole, and the percent of this that carbon makes up is (3(12)/58) X100 = 63%.

An unknown compound is found to contain 37.47% carbon, 12.61% hydrogen, and 49.92% oxygen by mass. What is the empirical formula for this compound?

CH4O

CH2O

C2H6O

C2H4O

Explanation

If we assume that we have a sample of 100g of the compound, then that would mean we have 37.47 grams (3.12 moles) of carbon, 12.61 grams (12.48 moles) of hydrogen, and 49.92 grams (3.12 moles) of oxygen.

The molar ratio of C:H:O is 3:12:3, reducing to 1:4:1, giving the empirical formula of CH4O.

A mystery carbohydrate is composed of 25% carbon, 8.3% hydrogen, and 66.7% oxygen. What is the molecular formula of this mystery carbohydrate, assuming the molar mass of the compound is three times as heavy as the molar mass of the empirical formula?

$C_{3}H_{12}O_{6}$

$C_{6}H_{24}O_{12}$

$CH_{4}O_{2}$

$C_{4}H_{16}O_{8}$

Explanation

In order to find the molecular formula, we must first find the empirical formula. We start by imagining a sample of the compound weighing 100 grams, so the percentages can be seen as grams. 25g of the sample is carbon, 8.3g of the sample is hydrogen, and 66.7g of the sample is oxygen.

Then, each of the atoms' masses is divided by the atom's molar mass, the numbers are then compared to each other in order to find the ratio of atoms.

$\frac{25g\ C}{12\frac{g}{mol}\ C}=2.08$ $\frac{8.3g\ H}{1\frac{g}{mol}\ H}=8.3$ $\frac{66.7g\ O}{16\frac{g}{mol}\ O}=4.16$

There is a ratio of two carbon to eight hydrogen to four oxygen, which reduces to a 1:4:2 ratio between carbon, hydrogen, and oxygen respectively.

As a result, the empirical formula is $CH_{4}O_{2}$ . Since the molecular formula is three times as heavy, we simply multiply each portion of the empirical formula by three. The molecular formula is $C_{3}H_{12}O_{6}$ .

What is the empirical formula for a sample that is 75% carbon by weight and 25% hydrogen by weight? (Note that this compound may not be realistic)

$C_{25}H_{100}$

$C_{6.25}H_{25}$

$C_{12.5}H_{50}$

None of the available answers

Explanation

To solve this problem, we will consider a sample that is 100g. We can assume that 75g of that sample is carbon and 25g of that sample is hydrogen.

First, we will determine the molar ratio of carbon to hydrogen.

$75g\ C\cdot\frac{1mol\ C}{12g\ C}=6.25mol\ C$

$25g\ H\cdot\frac{1mol\ H}{1g\ H}=25mol\ H$

Now we will determine the whole number ratio.

$C_{6.25}H_{25}=C_{25}H_{100}$

A compound is composed of 13% carbon, 4.3% hydrogen, 30.4% nitrogen, and 52.2% oxygen. The mystery compound has a molar mass of 184 grams per mole. What is the molecular formula of the compound?

$\small C_{2}H_{8}N_{4}O_{6}$

$\small CH_{4}N_{2}O_{3}$

$\small C_{2}H_{6}N_{4}O_{4}$

$\small C_{3}H_{12}N_{6}O_{9}$

Explanation

When finding molecular formulas, imagine a 100-gram sample of the compound. We can then use the percentages of each atom and convert them to grams.

$100g\ total$

$13g\ C,\ 4.3g\ H,\ 30.4g\ N,\ 52.2g\ O$

The next step is to divide the given mass of each atom by its atomic mass. This will give you four separate molar values that you can compare to one another.

$13g\ C*\frac{1mol}{12g}=1.08mol\ C$

$4.3g\ H*\frac{1mol}{1.0g}=4.3mol\ H$

$30.4g\ N*\frac{1mol}{14g}=2.17mol\ N$

$52.2g\ O*\frac{1mol}{16g}=3.26mol\ O$

Next, you must divide each molar value by the smallest of the values. This will result in the molar ratios of each compound. In this case, carbon has the lowest molar value at 1.08. After dividing all four numbers by this value, we determine a molar ratio of 1:4:2:3 for carbon, hydrogen, nitrogen, and oxygen, respectively.

$\frac{1.08}{1.08}:\frac{4.3}{1.08}:\frac{2.17}{1.08}:\frac{3.26}{1.08}$

At this point, we have determined the empirical formula for the compound, however, we need to find the formula that gives a molar mass of 184 grams per mole. Start by calculating the molar mass of the empirical formula.