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

$$\text{Mass Percent} = \frac{\text{mass of element in 1 mol of compound}}{\text{molar mass of compound}} \times 100\%$$

Q: Which of the following statements about the mole is correct?

A mole of any substance contains $$6.022 \times 10$$^{23}$$ particles.

Question 1

A compound is found to contain 40.0% carbon, 6.7% hydrogen, and 53.3% oxygen by mass. What is the empirical formula of this compound?

Accepted Answer

CH\_2O

Question 2

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

Accepted Answer

The empirical formula gives the simplest whole-number ratio of atoms in a compound, while the molecular formula gives the actual number of atoms of each element in a molecule.

Question 3

A sample of sodium chloride (NaCl) contains 71.1 kg. What is the number of formula units in this sample? (Molar mass NaCl = 58.44 g/mol, Avogadro's number $$6.022 	imes 10$$^{23}$$)

Accepted Answer

$$7.33 	imes 10$$^{26}$$

Question 4

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

Accepted Answer

C\_6H\_6

Question 5

Calculate the mass percent of carbon in sodium carbonate, Na\_2CO\_3. (Atomic masses: Na = 22.99, C = 12.01, O = 16.00)

Accepted Answer

11.3%

Question 6

A compound contains 4.19 	imes $$10^{22}$$ hydrogen atoms. If 10.0 g of the compound also contains 55.0% chlorine by mass, what is the empirical formula? (Atomic masses: H = 1.01, Cl = 35.45, C = 12.01)

Accepted Answer

CH\_2Cl

Question 7

If the empirical formula of a compound is CH\_2O and its molar mass is 180 g/mol, what is its molecular formula? (Empirical formula mass = 30 g/mol)

Accepted Answer

C\_6H\_12O\_6

Question 8

How many moles of chlorine gas are there in 8.33 	imes $$10^{22}$$ molecules of Cl\_2? (Avogadro's number $$6.022 	imes 10$$^{23}$$)

Accepted Answer

0.138 mol

Question 9

A pure compound is found to contain 72.5% C, 8.50% H, and the remainder O. If 0.250 moles of the compound weighs 41.5 g, what is the molecular formula? (Atomic masses: C = 12.01, H = 1.01, O = 16.00)

Accepted Answer

C\_6H\_12O\_2

Question 10

Which of the following is the correct formula to calculate mass percent of an element in a compound?

Accepted Answer

$$	ext{Mass Percent} = \frac{	ext{mass of element in 1 mol of compound}}{	ext{molar mass of compound}} 	imes 100\%$$

Question 11

If a sample of water contains 1.50 	imes $$10^{24}$$ molecules, how many moles of water are present? (Avogadro's number $$6.022 	imes 10$$^{23}$$)

Accepted Answer

2.49 mol

Question 12

A compound has an empirical formula of NO\_2 and a molar mass of 92 g/mol. What is its molecular formula? (Empirical formula mass = 46 g/mol)

Accepted Answer

N\_2O\_4

Question 13

Which step is NOT part of the process for determining the empirical formula from percent composition?

Accepted Answer

Divide the molar mass of the compound by the empirical formula mass.

Question 14

How many oxygen atoms are present in 783.9 g of Ni(NO\_3)\_2 \cdot 6H\_2O? (Molar mass = 290.7 g/mol)

Accepted Answer

$$9.77 	imes 10$$^{24}$$

Question 15

A cylindrical copper wire is used for a fence around a house. The copper wire has a diameter of 0.0750 in. and a length of 5.160 cm. If the density of copper is 8.96 $$g/cm^3$$, how many copper atoms are found in the wire? (Atomic mass of Cu = 63.55 g/mol, Avogadro's number $$6.022 	imes 10$$^{23}$$, 1$$\ 	ext{in} = 2.54\ 	ext{cm}$$)

Accepted Answer

$$1.10 	imes 10$$^{22}$$

Question 16

Which of the following statements about the mole is correct?

Accepted Answer

A mole of any substance contains $$6.022 	imes 10$$^{23}$$ particles.

Question 17

If the density of water is 1.00 g/mL at 25°C, what is the number of water molecules in 1.00 L of water? (Molar mass H\_2O = 18.02 g/mol, Avogadro's number $$6.022 	imes 10$$^{23}$$)

Accepted Answer

$$3.34 	imes 10$$^{25}$$

Chemical Composition: Empirical and Molecular Formulas, The Mole Concept, and Mass Percent

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