Q: A sample contains 0.551 g of potassium (K). How many atoms of potassium are present in the sample? ($$1\ \text{mol K} = 39.10\ \text{g K}$$, $$1\ \text{mol K} = 6.022 \times 10$$^{23}$$ atoms K)

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

Q: Which of the following is the correct calculation for the average atomic mass of lithium, given 7.42% $$^6Li (6.015 $$amu) and 92.58% $$^7Li (7.016 $$amu)?

$$\frac{(7.42 \times 6.015) + (92.58 \times 7.016)}{100}$$

Q: How many hydrogen atoms are present in 72.5 g of C$$_3H$_8O$$? (Molar mass of C$$_3H$_8O = 60 g/$$mol)

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

Q: Which of the following is the correct percent composition by mass of carbon in C$$_2H$_6O$$? (Molar mass of C$$_2H$_6O = 46.07 g/$$mol, atomic mass of C = 12.01 g/mol)

$$\frac{2 \times 12.01}{46.07} \times 100\% = 52.14\%$$

Q: Which of the following is the correct balanced equation for the combustion of ethane (C$$_2H$_6$$)?

$$2\text{C}_2\text{H}_6 + 7\text{O}_2 \rightarrow 4\text{CO}_2 + 6\text{H}_2\text{O}$$

Q: A chemical reaction has a theoretical yield of 250 g but only 200 g of product is actually obtained. What is the percent yield?

$$\frac{200}{250} \times 100 = 80\%$$

Q: Which of the following is the correct formula for calculating percent composition of an element in a compound?

$$\frac{n \times \text{molar mass of element}}{\text{molar mass of compound}} \times 100\%$$

Question 1

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

Accepted Answer

Atomic mass (amu) is numerically equal to the molar mass (grams) for any element.

Question 2

A sample contains 0.551 g of potassium (K). How many atoms of potassium are present in the sample? ($$1\ 	ext{mol K} = 39.10\ 	ext{g K}$$, $$1\ 	ext{mol K} = 6.022 	imes 10$$^{23}$$ atoms K)

Accepted Answer

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

Question 3

Which of the following is the correct calculation for the average atomic mass of lithium, given 7.42% $$^6Li (6.015 $$amu) and 92.58% $$^7Li (7.016 $$amu)?

Accepted Answer

$$\frac{(7.42 	imes 6.015) + (92.58 	imes 7.016)}{100}$$

Question 4

How many hydrogen atoms are present in 72.5 g of C$$_3H$_8O$$? (Molar mass of C$$_3H$_8O = 60 g/$$mol)

Accepted Answer

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

Question 5

Which of the following is the correct percent composition by mass of carbon in C$$_2H$_6O$$? (Molar mass of C$$_2H$_6O = 46.07 g/$$mol, atomic mass of C = 12.01 g/mol)

Accepted Answer

$$\frac{2 	imes 12.01}{46.07} 	imes 100\% = 52.14\%$$

Question 6

A student combusts 11.5 g of ethanol and collects 22.0 g CO$$_2$$ and 13.5 g H$$_2O. $$What is the empirical formula of ethanol based on the data?

Accepted Answer

C$$_2H$_6O$$

Question 7

Which of the following is the correct balanced equation for the combustion of ethane (C$$_2H$_6$$)?

Accepted Answer

$$2	ext{C}_2	ext{H}_6 + 7	ext{O}_2 ightarrow 4	ext{CO}_2 + 6	ext{H}_2	ext{O}$$

Question 8

Methanol burns in air according to the equation: $$2	ext{CH}_3	ext{OH} + 3	ext{O}_2 ightarrow 2	ext{CO}_2 + 4	ext{H}_2	ext{O}. If 209 g of $$methanol are used up, what mass of water is produced? (Molar mass CH$$_3OH = 32.0 g/$$mol, H$$_2O = 18.0 g/$$mol)

Accepted Answer

235 g

Question 9

In a reaction, 124 g of Al are reacted with 601 g of Fe$$_2O$_3$$ according to $$2	ext{Al} + 	ext{Fe}_2	ext{O}_3 ightarrow 	ext{Al}_2	ext{O}_3 + 2	ext{Fe}. $$What is the limiting reagent?

Accepted Answer

Al

Question 10

What is the theoretical yield of Al$$_2O$_3 (in $$grams) formed when 124 g of Al reacts with excess Fe$$_2O$_3$$? (Molar mass Al = 27.0 g/mol, Al$$_2O$_3 = 102 g/$$mol)

Accepted Answer

234 g

Question 11

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

Accepted Answer

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

Question 12

A chemical reaction has a theoretical yield of 250 g but only 200 g of product is actually obtained. What is the percent yield?

Accepted Answer

$$\frac{200}{250} 	imes 100 = 80\%$$

Question 13

Which of the following is Avogadro’s number?

Accepted Answer

$$6.022 	imes 10$$^{23}$$

Question 14

Which of the following best describes the process of balancing a chemical equation?

Accepted Answer

Adjusting the coefficients to ensure the same number of each type of atom on both sides of the equation.

Question 15

Which of the following is the correct formula for calculating percent composition of an element in a compound?

Accepted Answer

$$\frac{n 	imes 	ext{molar mass of element}}{	ext{molar mass of compound}} 	imes 100\%$$

Question 16

Which of the following is a real-world application of stoichiometry in agriculture?

Accepted Answer

Calculating the amount of ammonia needed to produce a specific mass of ammonium nitrate fertilizer.

Question 17

If you have 1 mole of SO$$_2$$, what is its mass in grams? (Atomic masses: S = 32.07, O = 16.00)

Accepted Answer

64.07 g

Question 18

Which of the following best explains why the mole is a useful counting unit in chemistry?

Accepted Answer

It allows chemists to count atoms and molecules by weighing macroscopic amounts of material.

Mass Relationships in Chemical Reactions: Stoichiometry and the Mole Concept

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