Question 1

Which statement best describes the Law of Conservation of Mass as it applies to chemical reactions in a closed system?

Accepted Answer

The total mass of the reactants must equal the total mass of the products.

Question 2

A student mixes 10 g of methane with 40 g of oxygen in a sealed container. After the reaction, the products are carbon dioxide and water vapor. According to the Law of Conservation of Mass, what is the total mass of the products?

Accepted Answer

$$50\ \mathrm{g}$$

Question 3

Which of the following best explains why you should never change subscripts when balancing chemical equations?

Accepted Answer

Changing subscripts changes the chemical identity of the substance.

Question 4

Given the unbalanced equation: $$\mathrm{CH_4$} + \mathrm{O_2$} \rightarrow \mathrm{CO_2$} + \mathrm{H_2O}$$, what are the correct coefficients to balance the equation?

Accepted Answer

$$1, 2, 1, 2$$

Question 5

A beaker containing a solution is placed on a balance and reads $$98.3\ \mathrm{g} ($$see image). If a chemical reaction occurs in the beaker and no mass is lost to the surroundings, what should the balance read after the reaction is complete?

Accepted Answer

$$98.3\ \mathrm{g}$$

Question 6

Which molecule is represented by the formula $$\mathrm{H_2O_2}$$, and why is it important not to confuse it with $$\mathrm{H_2O}$$?

Accepted Answer

Hydrogen peroxide; it is a bleach/poison, while $$\mathrm{H_2O} is $$safe to drink.

Question 7

When balancing the equation $$\mathrm{Na_2SO_4} + \mathrm{$$BaCl_2$$} \rightarrow \mathrm{$$BaSO_4$$} + \mathrm{2NaCl}$$, why is it helpful to treat $$\mathrm{SO_4$} as a $$single unit?

Accepted Answer

It simplifies the balancing process by reducing the number of elements to track.

Question 8

A student balances the equation $$\mathrm{H_2$} + \mathrm{O_2$} \rightarrow \mathrm{H_2O} by $$writing $$\mathrm{H_2$} + \mathrm{O_2$} \rightarrow \mathrm{H_2O_2}. $$What mistake did the student make?

Accepted Answer

They changed the subscript, creating a new substance.

Question 9

In an open system, a reaction produces a gas that escapes. The mass of the remaining substances appears to decrease. Which statement best explains this observation?

Accepted Answer

The law of conservation of mass still holds, but the escaped gas is not measured.

Question 10

Which of the following is the correct balanced equation for the combustion of methane ($$\mathrm{CH_4$}) in $$oxygen?

Accepted Answer

$$\mathrm{CH_4$} + 2\mathrm{O_2$} \rightarrow \mathrm{CO_2$} + 2\mathrm{H_2O}$$

Question 11

Why is it recommended to balance hydrogen and oxygen atoms last when balancing chemical equations?

Accepted Answer

They are often present in multiple compounds and balancing other elements first usually simplifies their balancing.

Question 12

A reaction starts with $$5\ \mathrm{g} of $$reactant A and $$15\ \mathrm{g} of $$reactant B in a closed flask. After the reaction, $$10\ \mathrm{g} of $$product C is collected. What is the mass of the other product formed?

Accepted Answer

$$10\ \mathrm{g}$$

Question 13

Which of the following best describes the difference between a coefficient and a subscript in a chemical equation?

Accepted Answer

A coefficient indicates the number of molecules; a subscript indicates the number of atoms of an element in a molecule.

Question 14

If the coefficients in a balanced equation are $$2:2:4$$, what should you do to simplify the equation?

Accepted Answer

Divide all coefficients by 2 to get $$1:1:2.$$

Question 15

A student claims that mass is lost during a chemical reaction because the mass measured after the reaction is less than before. What is the most likely explanation for this observation?

Accepted Answer

A gas produced in the reaction escaped from the system.

Conservation of Mass and Balancing Chemical Equations

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