Q: Which of the following is the correct formula to calculate the n-factor when determining a molecular formula?

$$n = \frac{\text{molar mass}}{\text{empirical mass}}$$

Q: Which of the following is the correct formula for calculating mass percent of an element in a compound?

$$\text{Mass Percent} = \frac{\text{mass of element}}{\text{mass of compound}} \times 100\%$$

Question 1

Which of the following best describes the empirical formula of a compound?

Accepted Answer

The most simplified whole-number ratio of atoms in a compound.

Question 2

A compound contains 68.40% chromium and 31.60% oxygen by mass. What is the first step in determining its empirical formula?

Accepted Answer

Write down the symbols for each element present.

Question 3

Which of the following is NOT a correct step in calculating the empirical formula from mass percent data?

Accepted Answer

Divide each mole value by the largest mole value.

Question 4

A compound has an empirical formula of CH2O and a molar mass of 180 g/mol. What is its molecular formula?

Accepted Answer

$$\mathrm{C}_6\mathrm{H}_{12}\mathrm{O}_6$$

Question 5

Which of the following is the correct formula to calculate the n-factor when determining a molecular formula?

Accepted Answer

$$n = \frac{	ext{molar mass}}{	ext{empirical mass}}$$

Question 6

Calculate the molar mass of $$\mathrm{(NH_4$)_2SO_4}$$ using the following atomic masses: N = 14.01 g/mol, H = 1.008 g/mol, S = 32.07 g/mol, O = 16.00 g/mol.

Accepted Answer

$$132.14\ \mathrm{g/mol}$$

Question 7

Which of the following compounds would NOT be associated with the term 'formula unit'?

Accepted Answer

$$\mathrm{BF_3$}$$

Question 8

How many molecules are in 2.00 moles of water ($$\mathrm{H_2O}$$)? (Avogadro's number = $$6.022 	imes 10$$^{23}$$)

Accepted Answer

$$1.20 	imes 10$$^{24}$$

Question 9

A sample contains $$9.25 	imes 10$$^{24}$$ molecules of $$\mathrm{$$Cl_2$$}$$. How many grams of $$\mathrm{$$Cl_2$$}$$ does this represent? (Molar mass of $$\mathrm{$$Cl_2$$}$$ = 70.90 g/mol)

Accepted Answer

$$1,088\ \mathrm{g}$$

Question 10

Which of the following best describes Avogadro’s number?

Accepted Answer

The number of particles in one mole of a substance.

Question 11

If a sample of sodium chloride ($$\mathrm{NaCl}) $$contains 73.1 kg, how many formula units are present? (Molar mass of $$\mathrm{NaCl} = 58.44 g/$$mol, Avogadro’s number = $$6.022 	imes 10$$^{23}$$)

Accepted Answer

$$7.54 	imes 10$$^{26}$$

Question 12

What is the mass percent of nitrogen in $$\mathrm{NO_2$}$$? (N = 14.01 g/mol, O = 16.00 g/mol)

Accepted Answer

$$30.45\%$$

Question 13

A compound contains 4.19 x $$10^{23}$$ hydrogen atoms in a 9.00 g sample, and is 55.0% chlorine by mass. What is the first step in determining its empirical formula?

Accepted Answer

Calculate the number of moles of each element present.

Question 14

Which of the following is the correct formula for calculating mass percent of an element in a compound?

Accepted Answer

$$	ext{Mass Percent} = \frac{	ext{mass of element}}{	ext{mass of compound}} 	imes 100\%$$

Question 15

A 0.05570 mol sample of diazepam (Valium) weighs 15.86 g. What is its molar mass?

Accepted Answer

$$285\ \mathrm{g/mol}$$

The Mole Concept: Empirical and Molecular Formulas, Molar Mass, and Mass Percent

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