Q: A sample contains $$2.5 \\ \times \\ 10$$^{21}$$ molecules of CO_2$. How many moles of CO_2$ are present in the sample? (Avogadro's number = 6.02$$ \\ \times \\ $$10^{23})$$

$$4.15 \\ \times \\ 10$$^{-3}$$ \\ \mathrm{mol}$$

Q: A bottle contains $$0.5 \\ \mathrm{mol} of$$ $$H_2O. $$How many molecules of water does it contain? (Avogadro's number $$= 6.02 \\ \times \\ 10$$^{23}$$)

$$3.01 \\ \times \\ 10$$^{23}$$

Q: A sample of $$NO_2$$ has a mass of $$92 \\ \mathrm{g}. If $$the molar mass of $$NO_2 is$$ $$46 \\ \mathrm{g/mol}$$, how many molecules are present in the sample? (Avogadro's number $$= 6.02 \\ \times \\ 10$$^{23}$$)

$$1.20 \\ \times \\ 10$$^{24}$$

Question 1

Which of the following best describes a compound?

Accepted Answer

A substance formed when two or more elements are chemically bonded in fixed proportions.

Question 2

Given that the molar mass of water ($$H_2O) is $$approximately $$18 \ \mathrm{g/mol}$$, what is the mass of $$1.7 \ \mathrm{mol} of $$water?

Accepted Answer

$$30.6 \ \mathrm{g}$$

Question 3

Which of the following statements about molecular mass is correct?

Accepted Answer

Molecular mass is the sum of the atomic masses of all atoms in a molecule, measured in atomic mass units (amu).

Question 4

A sample contains $$2.5 \ 	imes \ 10$$^{21}$$ molecules of CO_2$. How many moles of CO_2$ are present in the sample? (Avogadro's number = 6.02$$ \ 	imes \ $$10^{23})$$

Accepted Answer

$$4.15 \ 	imes \ 10$$^{-3}$$ \ \mathrm{mol}$$

Question 5

Which of the following best explains why the mass of a compound is not simply the sum of the masses of its elements in their natural forms?

Accepted Answer

Because the mass of a compound is the sum of the atomic masses of its constituent atoms, not the masses of the elements as found in nature.

Question 6

If you have $$68 \ \mathrm{g} of a $$compound with a molar mass of $$34 \ \mathrm{g/mol}$$, how many moles of the compound do you have?

Accepted Answer

$$2 \ \mathrm{mol}$$

Question 7

Which of the following is the correct formula to calculate the number of moles ($$n) $$from mass ($$m) $$and molar mass ($$M$$)?

Accepted Answer

$$n = \frac{m}{M}$$

Question 8

A chemist needs to prepare $$0.25 \ \mathrm{mol} of$$ $$NaCl$$ for an experiment. If the molar mass of $$NaCl is$$ $$58.5 \ \mathrm{g/mol}$$, what mass of $$NaCl$$ should be weighed out?

Accepted Answer

$$14.6 \ \mathrm{g}$$

Question 9

Which of the following best describes Avogadro's number?

Accepted Answer

The number of molecules in one mole of a substance.

Question 10

A student claims that $$1 \ \mathrm{mol} of$$ $$O_2$$ contains $$6.02 \ 	imes \ 10$$^{23}$$ oxygen atoms. Is this correct? Why or why not?

Accepted Answer

No, because $$O_2 is a $$molecule and one mole of $$O_2$$ contains $$1.204 \ 	imes \ 10$$^{24}$$ atoms.

Question 11

Which of the following is the correct way to calculate the molecular mass of $$CO_2$$?

Accepted Answer

Add the atomic mass of one carbon atom and two oxygen atoms.

Question 12

A bottle contains $$0.5 \ \mathrm{mol} of$$ $$H_2O. $$How many molecules of water does it contain? (Avogadro's number $$= 6.02 \ 	imes \ 10$$^{23}$$)

Accepted Answer

$$3.01 \ 	imes \ 10$$^{23}$$

Question 13

Which of the following is NOT a correct statement about chemical formulas?

Accepted Answer

They always indicate the physical state of the compound.

Question 14

If a compound has a formula mass of $$68 \ \mathrm{u}$$, what is the mass of $$2 \ \mathrm{mol} of $$this compound?

Accepted Answer

$$136 \ \mathrm{g}$$

Question 15

A sample of $$NO_2$$ has a mass of $$92 \ \mathrm{g}. If $$the molar mass of $$NO_2 is$$ $$46 \ \mathrm{g/mol}$$, how many molecules are present in the sample? (Avogadro's number $$= 6.02 \ 	imes \ 10$$^{23}$$)

Accepted Answer

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

Question 16

Which of the following best explains why water ($$H_2O) is $$considered a compound and not an element?

Accepted Answer

Because it can be separated into hydrogen and oxygen by chemical means.

Question 17

A student dissolves $$5.85 \ \mathrm{g} of$$ $$NaCl in $$water to make a solution. If the molar mass of $$NaCl is$$ $$58.5 \ \mathrm{g/mol}$$, what is the concentration of $$NaCl in $$moles if the final volume is $$0.5 \ \mathrm{L}$$?

Accepted Answer

$$0.1 \ \mathrm{mol/L}$$

Question 18

Which of the following is a correct application of the law of definite proportions?

Accepted Answer

A sample of $$CO_2$$ from any source will always have a mass ratio of carbon to oxygen of $$12:32.$$

Chemical Composition and Calculations in Compounds

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