Q: Which of the following is the correct formula to convert from mass to number of molecules?

$$\text{Number of molecules} = \frac{\text{mass (g)}}{\text{molar mass (g/mol)}} \times 6.02 \times 10$$^{23}$$

Q: If a sample of nitrogen gas (N$$_2) $$has a mass of 28.0 g, how many molecules are present in the sample? (Molar mass of N$$_2 = 28 g/$$mol, Avogadro's number = $$6.02 \times 10$$^{23}$$)

$$6.02 \times 10$$^{23}$$ molecules

Q: If you have 12.0 g of carbon (atomic mass = 12.0 g/mol), how many atoms of carbon do you have?

$$6.02 \times 10$$^{23}$$ atoms

Question 1

A sample of glucose (C$$_6H$_{12}O$_6) $$has a mass of 18.0 g. How many moles of glucose are present in the sample? (Molar mass of C$$_6H$_{12}O$_6 = 180 g/$$mol)

Accepted Answer

$$0.10$$ moles

Question 2

Which of the following best defines the mole in chemistry?

Accepted Answer

The amount of substance that contains as many entities as there are atoms in exactly 12 grams of carbon-12.

Question 3

A sample contains $$3.01 	imes 10$$^{23}$$ molecules of water. What is the mass of this sample? (Molar mass of H_2$O = 18 g/mol, Avogadro's number = 6.02$$ 	imes $$10^{23})$$

Accepted Answer

$$9.0 g$

Question 4

Which of the following is the correct formula to convert from mass to number of molecules?

Accepted Answer

$$	ext{Number of molecules} = \frac{	ext{mass (g)}}{	ext{molar mass (g/mol)}} 	imes 6.02 	imes 10$$^{23}$$

Question 5

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

Accepted Answer

CH$$_2O$$

Question 6

If a sample of nitrogen gas (N$$_2) $$has a mass of 28.0 g, how many molecules are present in the sample? (Molar mass of N$$_2 = 28 g/$$mol, Avogadro's number = $$6.02 	imes 10$$^{23}$$)

Accepted Answer

$$6.02 	imes 10$$^{23}$$ molecules

Question 7

A 50.0 g sample of a compound contains 20.0 g of sodium. What is the mass percent of sodium in the compound?

Accepted Answer

$$40.0\%$$

Question 8

A compound has an empirical formula of CH$$_2$$ and a molar mass of 56 g/mol. What is its molecular formula?

Accepted Answer

C$$_4H$_8$$

Question 9

Which of the following statements about molar mass is correct?

Accepted Answer

Molar mass is the mass in grams of one mole of a substance.

Question 10

A chemist analyzes a 10.0 g sample of a compound and finds it contains 2.0 g of hydrogen and 8.0 g of oxygen. What is the empirical formula of the compound?

Accepted Answer

H$$_2O$$

Question 11

If you have 12.0 g of carbon (atomic mass = 12.0 g/mol), how many atoms of carbon do you have?

Accepted Answer

$$6.02 	imes 10$$^{23}$$ atoms

Question 12

A sample of magnesium chloride (MgCl$$_2) $$weighs 95.0 g. What is the number of moles in the sample? (Molar mass of MgCl$$_2 = 95.0 g/$$mol)

Accepted Answer

$$1.00$$ mol

Question 13

A compound is found to contain 24.0 g of carbon, 4.0 g of hydrogen, and 32.0 g of oxygen. What is the empirical formula?

Accepted Answer

C$$_2H$_4O$_2$$

Question 14

Which of the following best describes how to use mass percent as a conversion factor?

Accepted Answer

Multiply the total mass of the compound by the mass percent (as a decimal) to find the mass of the element.

Question 15

A 25.0 g sample of a compound contains 10.0 g of element X. What is the mass percent of X in the compound?

Accepted Answer

$$40.0\%$$

Question 16

A compound has a molar mass of 58 g/mol and an empirical formula of C$$_2H$_5. $$What is its molecular formula?

Accepted Answer

C$$_4H$_{10}$$

Question 17

Which of the following is the correct sequence of steps to determine the empirical formula from mass percent composition?

Accepted Answer

Convert mass percent to grams, convert grams to moles, divide by smallest number of moles, write the formula.

Question 18

A 36.0 g sample of a compound contains 12.0 g of carbon, 2.0 g of hydrogen, and 22.0 g of oxygen. What is the empirical formula?

Accepted Answer

CH$$_2O$$

Question 19

If you have $$1.50 	imes 10$$^{24}$$ molecules of CO_2$, how many moles of CO_2$ do you have? (Avogadro's number = 6.02$$ 	imes $$10^{23})$$

Accepted Answer

Question 20

A 100.0 g sample of a compound contains 27.3 g of carbon, 72.7 g of oxygen. What is the empirical formula?

Accepted Answer

CO$$_2$$

Chemical Composition: The Mole, Molar Mass, and Empirical Formulas

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