Q: A sample contains 3.01 $\times$ 10$^{23}$ molecules of water. How many moles of water are present?

$\frac{3.01 \times 10^{23}}{6.022 \times 10^{23}} = 0.500\ \text{mol}$

Q: What is the percent composition by mass of oxygen in H$_2$O? (Atomic weights: H = 1.01, O = 16.00)

$\frac{16.00}{2 \times 1.01 + 16.00} \times 100\% = 88.8\%$

Q: Which of the following is the correct formula for calculating the number of moles from mass and molar mass?

$\text{moles} = \frac{\text{mass (g)}}{\text{molar mass (g/mol)}}$

Q: How many atoms are present in 2.00 moles of helium?

$2.00 \times 6.022 \times 10^{23} = 1.204 \times 10^{24}$ atoms

Q: Which of the following is the correct formula for percent composition by mass?

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

Q: A sample of nitrogen gas has a mass of 28.0 g. How many moles of nitrogen molecules (N$_2$) are present? (Molar mass of N$_2$ = 28.0 g/mol)

$\frac{28.0}{28.0} = 1.00\ \text{mol}$

Question 1

Which of the following statements best describes the difference between a compound and a mixture?

Accepted Answer

A compound consists of two or more elements chemically bonded in fixed proportions, while a mixture contains two or more substances physically combined in variable proportions.

Question 2

A student measures the mass of a metal sample as 12.45 g and its volume as 2.50 cm$^3$. What is the density of the metal?

Accepted Answer

$\frac{12.45}{2.50} = 4.98\ \mathrm{g/cm^3}$

Question 3

Which of the following is the correct conversion factor to change Celsius temperature to Kelvin?

Accepted Answer

$K = ^\circ C + 273.15$

Question 4

Which of the following is a correct example of scientific notation for the number 0.00056?

Accepted Answer

$5.6 	imes 10^{-4}$

Question 5

A sample contains 3.01 $	imes$ 10$^{23}$ molecules of water. How many moles of water are present?

Accepted Answer

$\frac{3.01 	imes 10^{23}}{6.022 	imes 10^{23}} = 0.500\ 	ext{mol}$

Question 6

Which of the following best describes an isotope?

Accepted Answer

Atoms of the same element with different numbers of neutrons.

Question 7

Which of the following elements is a metalloid?

Accepted Answer

Silicon (Si)

Question 8

What is the percent composition by mass of oxygen in H$_2$O? (Atomic weights: H = 1.01, O = 16.00)

Accepted Answer

$\frac{16.00}{2 	imes 1.01 + 16.00} 	imes 100\% = 88.8\%$

Question 9

Which of the following is the correct empirical formula for glucose (C$_6$H$_{12}$O$_6$)?

Accepted Answer

CH$_2$O

Question 10

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

Accepted Answer

CH$_2$O

Question 11

Which of the following ions is a polyatomic ion?

Accepted Answer

SO$_4^{2-}$

Question 12

Which of the following is the correct formula for calculating the number of moles from mass and molar mass?

Accepted Answer

$	ext{moles} = \frac{	ext{mass (g)}}{	ext{molar mass (g/mol)}}$

Question 13

Which of the following best describes a chemical change?

Accepted Answer

A process in which substances are transformed into new substances with different chemical properties.

Question 14

How many atoms are present in 2.00 moles of helium?

Accepted Answer

$2.00 	imes 6.022 	imes 10^{23} = 1.204 	imes 10^{24}$ atoms

Question 15

Which of the following is the correct formula for percent composition by mass?

Accepted Answer

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

Question 16

Which of the following is the correct name for the compound Na$_2$SO$_4$?

Accepted Answer

Sodium sulfate

Question 17

A sample of nitrogen gas has a mass of 28.0 g. How many moles of nitrogen molecules (N$_2$) are present? (Molar mass of N$_2$ = 28.0 g/mol)

Accepted Answer

$\frac{28.0}{28.0} = 1.00\ 	ext{mol}$

Question 18

Which of the following is the correct formula for calculating the number of molecules in a given number of moles?

Accepted Answer

$	ext{number of molecules} = 	ext{moles} 	imes 6.022 	imes 10^{23}$

Fundamental Concepts in General Chemistry: Elements, Compounds, Atomic Structure, and Chemical Formulas

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