Q: A sample contains 0.551 g of potassium ($$K). $$How many atoms of potassium are present in the sample? ($$M_{K} = 39.10\ \mathrm{g/mol})$$

$$8.49 \times 10$$^{21}$$ atoms

Q: Which of the following is the correct formula to calculate the average atomic mass of an element with two isotopes?

$$\text{Average atomic mass} = (\text{fraction}_1 \times \text{mass}_1) + (\text{fraction}_2 \times \text{mass}_2)$$

Q: How many hydrogen atoms are present in 72.5 g of $$\mathrm{C_3H_8O}$$? ($$M_{\mathrm{C_3H_8O}} = 60\ \mathrm{g/mol})$$

$$5.82 \times 10$$^{24}$$ atoms

Question 1

Which of the following statements best describes the mole (mol) in chemistry?

Accepted Answer

It is the amount of a substance that contains as many entities as there are atoms in exactly 12.00 grams of $$^{12}\mathrm{C}.$$

Question 2

A sample contains 0.551 g of potassium ($$K). $$How many atoms of potassium are present in the sample? ($$M_{K} = 39.10\ \mathrm{g/mol})$$

Accepted Answer

$$8.49 	imes 10$$^{21}$$ atoms

Question 3

Which of the following is the correct formula to calculate the average atomic mass of an element with two isotopes?

Accepted Answer

$$	ext{Average atomic mass} = (	ext{fraction}_1 	imes 	ext{mass}_1) + (	ext{fraction}_2 	imes 	ext{mass}_2)$$

Question 4

How many hydrogen atoms are present in 72.5 g of $$\mathrm{C_3H_8O}$$? ($$M_{\mathrm{C_3H_8O}} = 60\ \mathrm{g/mol})$$

Accepted Answer

$$5.82 	imes 10$$^{24}$$ atoms

Question 5

What is the empirical formula of a compound with the following percent composition by mass: K 24.75%, Mn 34.77%, O 40.51%?

Accepted Answer

$$\mathrm{$$KMnO_4$$}$$

Question 6

A 10.0 g sample of HCl contains how many grams of hydrogen?

Accepted Answer

$$0.276 g$$

Question 7

Which of the following best describes the limiting reagent in a chemical reaction?

Accepted Answer

The reactant that is completely consumed first, limiting the amount of product formed.

Question 8

In the reaction $$2\mathrm{Al} + \mathrm{Fe_2O_3} \rightarrow \mathrm{Al_2O_3} + 2\mathrm{Fe}$$, if 124 g of Al and 601 g of $$\mathrm{Fe_2O_3}$$ are reacted, what is the mass of $$\mathrm{Al_2O_3}$$ formed? ($$M_{\mathrm{Al}} = 26.98\ \mathrm{g/mol}$$, $$M_{\mathrm{Fe_2O_3}} = 159.7\ \mathrm{g/mol}$$, $$M_{\mathrm{Al_2O_3}} = 102.0\ \mathrm{g/mol})$$

Accepted Answer

234.4 g

Question 9

A compound has an empirical formula of $$\mathrm{CH_2O}$$ and a molar mass of 180 g/mol. What is its molecular formula?

Accepted Answer

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

Question 10

What is the percent composition by mass of carbon in ethanol ($$\mathrm{C_2H_6O}$$, molar mass = 46.07 g/mol)?

Accepted Answer

52.14%

Question 11

Which of the following is the correct balanced equation for the combustion of ethane ($$\mathrm{C_2H_6}$$)?

Accepted Answer

$$2\mathrm{C_2H_6} + 7\mathrm{O_2$} \rightarrow 4\mathrm{CO_2$} + 6\mathrm{H_2O}$$

Question 12

If 209 g of methanol ($$\mathrm{CH_3OH}$$, $$M = 32.0\ \mathrm{g/mol}) $$are combusted, what mass of water is produced? (Balanced equation: $$2\mathrm{CH_3OH} + 3\mathrm{O_2$} \rightarrow 2\mathrm{CO_2$} + 4\mathrm{H_2O})$$

Accepted Answer

235 g

Question 13

A sample of ascorbic acid (vitamin C) contains 40.92% C, 4.58% H, and 54.50% O by mass. What is its empirical formula?

Accepted Answer

Question 14

What is the percent yield if the theoretical yield of $$\mathrm{CaO} is 2374.2 g $$and the actual yield is 803 g?

Accepted Answer

33.8%

Question 15

Which of the following best describes the relationship between atomic mass (amu) and molar mass (grams)?

Accepted Answer

The atomic mass in amu is numerically equal to the molar mass in grams for one mole of atoms.

Question 16

A mass spectrometer is used to determine the isotopic composition of neon. If the spectrum shows peaks at masses 20, 21, and 22, what does this indicate?

Accepted Answer

Neon has three naturally occurring isotopes with mass numbers 20, 21, and 22.

Question 17

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

Accepted Answer

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

Mass Relationships in Chemical Reactions: Atomic Mass, Mole Concept, and Stoichiometry

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