Question 1

Which statement best describes the difference between a molecular formula and an empirical formula?

Accepted Answer

The molecular formula shows the actual number of atoms in a molecule, while the empirical formula shows the simplest whole-number ratio of atoms.

Question 2

A compound contains 68.40% chromium and 31.60% oxygen by mass. What is the empirical formula of this compound? (Atomic masses: Cr = 52.00, O = 16.00)

Accepted Answer

$$\mathrm{Cr_2O_3}$$

Question 3

Which step is necessary when calculating the empirical formula from percent composition?

Accepted Answer

Convert percentages to grams by assuming a 100 g sample.

Question 4

A compound contains 48.64% C, 43.21% O, and 8.15% H by mass. What is the empirical formula? (Atomic masses: C = 12.01, H = 1.01, O = 16.00)

Accepted Answer

$$\mathrm{C_2H_5O}$$

Question 5

Elemental analysis of a sample of an ionic compound showed 2.82 g Na, 4.35 g Cl, and 7.83 g O. What is the empirical formula? (Atomic masses: Na = 22.99, Cl = 35.45, O = 16.00)

Accepted Answer

$$\mathrm{$$NaClO_3$$}$$

Question 6

If a compound contains 55.05% chlorine by mass and 4.19 	imes $$10^{23}$$ hydrogen atoms in a 9.00 g sample, what is the empirical formula? (Atomic masses: C = 12.01, H = 1.01, Cl = 35.45)

Accepted Answer

$$\mathrm{C_2HCl}$$

Question 7

Why is it important to round the mole ratios to the nearest whole number when determining the empirical formula?

Accepted Answer

Because atoms combine in whole-number ratios in compounds.

Question 8

A compound is found to contain 40.0% carbon, 6.7% hydrogen, and 53.3% oxygen by mass. What is the empirical formula? (Atomic masses: C = 12.01, H = 1.01, O = 16.00)

Accepted Answer

$$\mathrm{CH_2O}$$

Question 9

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

Accepted Answer

Multiply the mole ratios by the atomic number of each element.

Question 10

A sample contains 0.90 g of hydrogen and 7.20 g of oxygen. What is the empirical formula? (Atomic masses: H = 1.01, O = 16.00)

Accepted Answer

$$\mathrm{H_2O}$$

Question 11

If the empirical formula of a compound is $$\mathrm{CH_2O}$$ and its molar mass is 180 g/mol, what is its molecular formula? (Empirical formula mass = 30 g/mol)

Accepted Answer

$$\mathrm{C_6H_{12}O_6$}$$

Question 12

A compound contains 25.9% nitrogen and 74.1% oxygen by mass. What is the empirical formula? (Atomic masses: N = 14.01, O = 16.00)

Accepted Answer

$$\mathrm{N_2O_5}$$

Question 13

Which of the following best explains why empirical formulas are useful in chemistry?

Accepted Answer

They provide the simplest ratio of elements, which is useful for identifying unknown compounds.

Question 14

A compound contains 36.5% sodium, 25.4% sulfur, and 38.1% oxygen by mass. What is the empirical formula? (Atomic masses: Na = 22.99, S = 32.07, O = 16.00)

Accepted Answer

$$\mathrm{Na_2SO_4}$$

Question 15

Which of the following compounds has the same empirical formula as glucose ($$\mathrm{C_6H_{12}O_6$}$$)?

Accepted Answer

$$\mathrm{C_2H_4O_2}$$

Question 16

A compound contains 53.5% carbon, 15.5% hydrogen, and 31.0% nitrogen by mass. What is the empirical formula? (Atomic masses: C = 12.01, H = 1.01, N = 14.01)

Accepted Answer

$$\mathrm{C_3H_{10}N_2$}$$

Question 17

If a compound's empirical formula is $$\mathrm{CH}$$ and its molecular mass is 78 g/mol, what is its molecular formula? (Empirical formula mass = 13 g/mol)

Accepted Answer

$$\mathrm{C_6H_6}$$

Question 18

A compound contains 92.3% carbon and 7.7% hydrogen by mass. What is the empirical formula? (Atomic masses: C = 12.01, H = 1.01)

Accepted Answer

$$\mathrm{C_3H_2}$$

Question 19

Which of the following is a correct reason for using the empirical formula in medical chemistry?

Accepted Answer

It helps identify the basic composition of drugs and their active ingredients.

Empirical and Molecular Formulas: Calculation and Practice

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