Q: A sample contains $$0.551\ \mathrm{g} of $$potassium ($$\mathrm{K}). $$How many atoms of potassium are present? ($$\mathrm{Molar\ mass\ of\ K} = 39.10\ \mathrm{g/mol}$$, $$N_A = 6.022 \times 10$$^{23}$$)

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

Q: What is Avogadro's number and what does it represent?

$$6.022 \times 10$$^{23}$$, the number of molecules in one mole of a substance

Question 1

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

Accepted Answer

The total mass of reactants equals the total mass of products in a chemical reaction.

Question 2

Why do chemists balance chemical equations by changing coefficients rather than subscripts?

Accepted Answer

Changing coefficients alters the amount of substances without changing their identity.

Question 3

Given the balanced equation $$2 \mathrm{Mg} + \mathrm{O}_2 \rightarrow 2 \mathrm{MgO}$$, how many moles of MgO are produced from 6 moles of Mg?

Accepted Answer

6 moles

Question 4

Which of the following is the correct molar mass of $$\mathrm{NaCl}$$?

Accepted Answer

$$58.44\ \mathrm{g/mol}$$

Question 5

A sample contains $$0.551\ \mathrm{g} of $$potassium ($$\mathrm{K}). $$How many atoms of potassium are present? ($$\mathrm{Molar\ mass\ of\ K} = 39.10\ \mathrm{g/mol}$$, $$N_A = 6.022 	imes 10$$^{23}$$)

Accepted Answer

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

Question 6

Which of the following is a decomposition reaction?

Accepted Answer

$$2 \mathrm{NaN}_3 \rightarrow 2 \mathrm{Na} + 3 \mathrm{N}_2$$

Question 7

What is the percent composition by mass of oxygen in $$\mathrm{CO}_2$$? ($$\mathrm{C} = 12.01\ \mathrm{g/mol}$$, $$\mathrm{O} = 16.00\ \mathrm{g/mol})$$

Accepted Answer

$$72.7\%$$

Question 8

Which of the following best describes a combustion reaction?

Accepted Answer

A rapid reaction involving oxygen as a reactant, producing $$\mathrm{CO}_2$$ and $$\mathrm{H}_2\mathrm{O}$$ from hydrocarbons.

Question 9

Calculate the formula mass of $$\mathrm{Ca}_3(\mathrm{PO}_4)_2$$ using the following atomic masses: $$\mathrm{Ca} = 40.08$$, $$\mathrm{P} = 30.97$$, $$\mathrm{O} = 16.00.$$

Accepted Answer

$$310.18\ \mathrm{amu}$$

Question 10

A compound contains $$40.92\% C$$, $$4.58\% H$$, and $$54.50\% O by $$mass. What is its empirical formula?

Accepted Answer

$$\mathrm{CH}_2\mathrm{O}$$

Question 11

Which of the following is the correct sequence of steps for determining the empirical formula from percent composition?

Accepted Answer

Convert mass to moles, divide by smallest number of moles, multiply to get whole numbers.

Question 12

If $$2.00\ \mathrm{mol} of$$ $$\mathrm{CH}_4$$ reacts with excess $$\mathrm{O}_2$$ according to $$\mathrm{CH}_4 + 2 \mathrm{O}_2 \rightarrow \mathrm{CO}_2 + 2 \mathrm{H}_2\mathrm{O}$$, how many moles of $$\mathrm{H}_2\mathrm{O}$$ are produced?

Accepted Answer

$$4.00\ \mathrm{mol}$$

Question 13

Which of the following is the correct relationship between atomic mass and molar mass for any element?

Accepted Answer

Atomic mass (amu) and molar mass (g/mol) are numerically equal.

Question 14

How many moles are present in $$80.6\ \mathrm{g} of$$ $$\mathrm{MgO}$$? ($$\mathrm{MgO}$$ molar mass = $$40.3\ \mathrm{g/mol})$$

Accepted Answer

$$2.00\ \mathrm{mol}$$

Question 15

Which of the following is NOT a combination reaction?

Accepted Answer

$$2 \mathrm{NaN}_3 \rightarrow 2 \mathrm{Na} + 3 \mathrm{N}_2$$

Question 16

What is Avogadro's number and what does it represent?

Accepted Answer

$$6.022 	imes 10$$^{23}$$, the number of molecules in one mole of a substance

Question 17

A student mixes $$10.0\ \mathrm{g} of$$ $$\mathrm{CH}_4$$ with excess $$\mathrm{O}_2$$ and ignites the mixture. How many grams of $$\mathrm{CO}_2$$ are produced? ($$\mathrm{CH}_4$$ molar mass = $$16.0\ \mathrm{g/mol}$$, $$\mathrm{CO}_2$$ molar mass = $$44.0\ \mathrm{g/mol})$$

Accepted Answer

$$27.5\ \mathrm{g}$$

Question 18

Which symbol in a chemical equation indicates that a substance is dissolved in water?

Accepted Answer

$$(aq)$$

Chemical Reactions and Reaction Stoichiometry (CHEM 200, Chapter 3)

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