Q: Which of the following is the correct formula to calculate percent yield?

$$\text{Percent yield} = \frac{\text{actual yield}}{\text{theoretical yield}} \times 100$$

Question 1

Which of the following best describes the purpose of heating the crucible before adding magnesium in the experiment?

Accepted Answer

To remove moisture and ensure accurate mass measurements

Question 2

Given the reaction $$2 \mathrm{Mg} + \mathrm{O}_2 \rightarrow 2 \mathrm{MgO}$$, what is the theoretical yield of MgO if 0.500 g of Mg is used? (Molar mass of Mg = 24.3 g/mol, MgO = 40.3 g/mol)

Accepted Answer

$$0.830\ \mathrm{g}$$

Question 3

In the experiment, why is it important to use excess oxygen when reacting magnesium?

Accepted Answer

To ensure all magnesium is converted to magnesium oxide

Question 4

If 0.482 g of MgO is obtained from 0.500 g of Mg, what is the percent yield of MgO? (Theoretical yield from previous calculation is 0.830 g)

Accepted Answer

$$58.1\%$$

Question 5

Which equation correctly represents the reaction of magnesium with nitrogen in air to form magnesium nitride?

Accepted Answer

$$3\ \mathrm{Mg} + \mathrm{N}_2 \rightarrow \mathrm{Mg}_3\mathrm{N}_2$$

Question 6

During the experiment, magnesium nitride ($$\mathrm{Mg}_3\mathrm{N}_2) $$can form if oxygen is limited. What is the purpose of adding water after the reaction?

Accepted Answer

To convert magnesium nitride to magnesium hydroxide and ammonia

Question 7

Which of the following is the correct formula to calculate percent yield?

Accepted Answer

$$	ext{Percent yield} = \frac{	ext{actual yield}}{	ext{theoretical yield}} 	imes 100$$

Question 8

A student obtained a percent yield greater than 100% for MgO. Which of the following is the most likely explanation?

Accepted Answer

The product was contaminated with moisture or impurities

Question 9

If 0.250 g of Mg reacts with excess oxygen, how many moles of MgO are theoretically produced? (Molar mass of Mg = 24.3 g/mol, MgO = 40.3 g/mol)

Accepted Answer

$$0.0103\ \mathrm{mol}$$

Question 10

Which of the following steps is necessary to ensure accurate mass measurements in the experiment?

Accepted Answer

Cooling the crucible in a desiccator before weighing

Question 11

In the reaction $$2\ \mathrm{Al} + 3\ \mathrm{O}_2 \rightarrow 2\ \mathrm{Al}_2\mathrm{O}_3$$, what is the limiting reactant if 0.500 g of Al and 1.00 g of $$\mathrm{O}_2$$ are used? (Molar mass: Al = 27.0 g/mol, $$\mathrm{O}_2 = 32.0 g/$$mol)

Accepted Answer

Aluminum ($$\mathrm{Al})$$

Question 12

Why is it important to use a lid with the crucible during the heating of magnesium?

Accepted Answer

To prevent loss of magnesium oxide by splattering

Question 13

Which of the following best explains why magnesium nitride forms during the experiment?

Accepted Answer

Magnesium reacts with nitrogen when oxygen is limited

Question 14

If a student fails to heat the crucible to constant mass before starting the experiment, what error could result?

Accepted Answer

The mass of the crucible may include moisture, leading to inaccurate yield calculations

Question 15

Which of the following is a correct balanced equation for the conversion of magnesium nitride to magnesium hydroxide and ammonia?

Accepted Answer

$$\mathrm{Mg}_3\mathrm{N}_2 + 6\ \mathrm{H}_2\mathrm{O} \rightarrow 3\ \mathrm{Mg(OH)}_2 + 2\ \mathrm{NH}_3$$

Question 16

A student observes a white solid after heating magnesium in air. Which product is most likely formed?

Accepted Answer

Magnesium oxide ($$\mathrm{MgO})$$

Question 17

If the mass of the crucible, lid, and Mg before reaction is 25.000 g, and after reaction (with MgO) is 25.830 g, what is the mass of MgO formed?

Accepted Answer

$$0.830\ \mathrm{g}$$

Question 18

Which of the following is a reason to test for completeness of reaction with hydrochloric acid after heating magnesium?

Accepted Answer

To check for the presence of unreacted magnesium

Question 19

In a real-world scenario, why is it important to calculate the theoretical yield before performing a chemical reaction?

Accepted Answer

To predict the maximum amount of product and plan resource usage

Quantities in Chemical Reactions: Stoichiometry and Yield Calculations

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