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

$$\frac{(\text{number of atoms})(\text{atomic mass})}{\text{formula weight of compound}} \times 100$$

Question 1

Which law is illustrated by the balanced scale in the image, where the number of atoms on both sides of a chemical equation is equal?

Accepted Answer

Law of Conservation of Mass

Question 2

Given the unbalanced equation: $$\mathrm{2NaN_3 \rightarrow Na + 3N_2}$$, what is the main reason this equation is not balanced?

Accepted Answer

The number of sodium atoms is not equal on both sides.

Question 3

Which of the following best describes the difference between coefficients and subscripts in a chemical equation?

Accepted Answer

Coefficients indicate the number of molecules; subscripts indicate the number of atoms in a molecule.

Question 4

A reaction is represented as $$\mathrm{C_3H_8 + 5O_2 \rightarrow 3CO_2 + 4H_2O}. If 150 g of$$ $$\mathrm{CO_2$} is $$produced, what mass of propane ($$\mathrm{C_3H_8}) $$was combusted? (Molar masses: $$\mathrm{CO_2$} = 44.01 g/$$mol, $$\mathrm{C_3H_8} = 44.09 g/$$mol)

Accepted Answer

50.1 g

Question 5

Which of the following is true about the reaction $$\mathrm{4NH_3(g) + 5O_2(g) \rightarrow 4NO(g) + 6H_2O(g)}$$?

Accepted Answer

The number of nitrogen, hydrogen, and oxygen atoms are all balanced.

Question 6

How many molecules are in 0.0861 moles of $$\mathrm{CO_2$}$$? (Avogadro’s number = $$6.02 	imes 10$$^{23}$$)

Accepted Answer

$$5.18 	imes 10$$^{22}$$

Question 7

What is the molar mass of $$\mathrm{Ca(NO_3$)_2}$$?

Accepted Answer

164.1 g/mol

Question 8

A solution is prepared by dissolving 0.50 moles of solute in 250 mL of solution. What is the molarity of the solution?

Accepted Answer

2.0 M

Question 9

What is the percent by mass of NaCl if 25.5 g of it is dissolved in 425 g of water?

Accepted Answer

5.66%

Question 10

Which of the following best explains why the volume of 1 mole of any gas at STP is always 22.4 L?

Accepted Answer

Avogadro’s hypothesis states that equal volumes of gases at the same temperature and pressure contain the same number of molecules.

Question 11

How many grams of water are produced when 100. g of $$\mathrm{NH_3$}$$ are fully reacted with oxygen? (Molar mass $$\mathrm{NH_3$} = 17.0 g/$$mol, $$\mathrm{H_2O} = 18.0 g/$$mol, equation: $$4NH_3 + 5O_2 \rightarrow 4NO + 6H_2O)$$

Accepted Answer

159 g

Question 12

Which of the following is NOT a correct way to balance a chemical equation?

Accepted Answer

Changing the subscripts within a chemical formula

Question 13

A solution is made by dissolving 18.0 g of $$\mathrm{H_3PO_4} in 2.00 L of $$solution. What is the molarity? (Molar mass $$\mathrm{H_3PO_4} = 98.0 g/$$mol)

Accepted Answer

0.092 M

Question 14

What is the percent by volume of oil in a mixture of 120 mL oil and 4.0 L of fuel?

Accepted Answer

3.0%

Question 15

If 0.344 moles of $$\mathrm{$$KNO_3$$}$$ are dissolved in 0.250 L of solution, what is the molarity?

Accepted Answer

1.38 M

Question 16

Which of the following best describes a solution with high concentration?

Accepted Answer

It contains a large amount of solute relative to the solvent.

Question 17

How many moles of $$\mathrm{CO_2$}$$ are produced when 7.5 moles of $$\mathrm{O_2$}$$ are consumed in the combustion of propane? ($$\mathrm{C_3H_8 + 5O_2 \rightarrow 3CO_2 + 4H_2O})$$

Accepted Answer

4.5 moles

Question 18

What is the mass in grams of 0.400 mol $$\mathrm{CH_4$}$$? (Molar mass $$\mathrm{CH_4$} = 16.0 g/$$mol)

Accepted Answer

6.4 g

Question 19

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

Accepted Answer

$$\frac{(	ext{number of atoms})(	ext{atomic mass})}{	ext{formula weight of compound}} 	imes 100$$

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