Q: Which equation correctly represents the calculation for percent yield?

$$\text{Percent yield} = \frac{\text{Actual yield}}{\text{Theoretical yield}} \times 100\%$$

Question 1

Which of the following best describes the process for converting the mass of one reactant to the mass of a product in a chemical reaction?

Accepted Answer

Convert mass of reactant to moles, use stoichiometry to find moles of product, then convert moles of product to mass.

Question 2

Given the reaction $$2	ext{Na}(s) + 	ext{Cl}_2(g) ightarrow 2	ext{NaCl}(s)$$, if you start with 53.2 g Na and 65.8 g Cl$$_2$$, which is the limiting reactant? (Molar masses: Na = 22.99 g/mol, Cl$$_2 = 70.90 g/$$mol)

Accepted Answer

Na

Question 3

For the reaction $$2	ext{Na}(s) + 	ext{Cl}_2(g) ightarrow 2	ext{NaCl}(s)$$, calculate the theoretical yield of NaCl (in grams) if you start with 53.2 g Na and excess Cl$$_2. ($$Molar mass NaCl = 58.44 g/mol)

Accepted Answer

$$135.1\ 	ext{g}$$

Question 4

If the actual yield of NaCl in the reaction $$2	ext{Na}(s) + 	ext{Cl}_2(g) ightarrow 2	ext{NaCl}(s) is 86.6 g$$, and the theoretical yield is 135.1 g, what is the percent yield?

Accepted Answer

$$64.1\%$$

Question 5

Which of the following statements about the limiting reactant is correct?

Accepted Answer

It determines the maximum amount of product that can be formed.

Question 6

A reaction has a $$\Delta H_{	ext{RXN}} = -285.8\ 	ext{kJ}$$ for the combustion of 2 mol H$$_2. $$How much heat is released when 4.00 g of H$$_2 is $$combusted? (Molar mass H$$_2 = 2.02 g/$$mol)

Accepted Answer

$$-282\ 	ext{kJ}$$

Question 7

Which equation correctly represents the calculation for percent yield?

Accepted Answer

$$	ext{Percent yield} = \frac{	ext{Actual yield}}{	ext{Theoretical yield}} 	imes 100\%$$

Question 8

In a reaction, 10.0 g of A reacts with 15.0 g of B to produce 12.0 g of C. If the theoretical yield of C is 14.0 g, what is the percent yield?

Accepted Answer

$$85.7\%$$

Question 9

Which of the following best describes an exothermic reaction?

Accepted Answer

A reaction that releases heat to the surroundings ($$\Delta H_{	ext{RXN}} \u003c 0)$$

Question 10

A chemical reaction has $$\Delta H_{	ext{RXN}} = +125\ 	ext{kJ}$$ for the reaction as written. What does the positive sign indicate?

Accepted Answer

The reaction absorbs heat from the surroundings.

Question 11

If you have 10.0 g of reactant A (molar mass = 20.0 g/mol) and it reacts according to the equation $$A + 2B \rightarrow C$$, how many moles of A do you have?

Accepted Answer

$$0.500\ 	ext{mol}$$

Question 12

Which of the following is NOT a step in a typical mass-to-mass stoichiometry problem?

Accepted Answer

Multiply the mass of reactant by the molar mass of the product.

Question 13

A reaction has a theoretical yield of 50.0 g and an actual yield of 45.0 g. What is the percent yield?

Accepted Answer

$$90.0\%$$

Question 14

In a reaction, if the actual yield is greater than the theoretical yield, what is the most likely explanation?

Accepted Answer

The product was contaminated with impurities.

Question 15

Which of the following is true about the theoretical yield?

Accepted Answer

It is calculated based on the limiting reactant.

Stoichiometry and Quantities in Chemical Reactions

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