Calculating Mass of HCl Required

Oct 13, 2025 by talynt.org 23 views

Introduction

Chemical reactions are an essential part of chemistry, and understanding the principles behind them is crucial for any student of the subject. In this article, we will explore the reaction between magnesium (Mg) and hydrochloric acid (HCl), and use the given equation to determine the correct answer to a question posed by a student.

The Reaction Equation

The reaction between magnesium and hydrochloric acid is given by the following equation:

${ Mg_{(s)} + 2 HCl_{(aq)} \rightarrow MgCl_{2(aq)} + H_{2(g)} }$

This equation tells us that when magnesium reacts with hydrochloric acid, it produces magnesium chloride (MgCl2) and hydrogen gas (H2).

The Question

A student placed 3.0 grams of Mg into some HCl in two different experiments. In each case, it reacted according to the given equation. The question is: what is the limiting reactant in each experiment?

Limiting Reactant

To determine the limiting reactant, we need to calculate the number of moles of Mg and HCl in each experiment. We can then compare the mole ratio of Mg to HCl to determine which reactant is limiting.

Calculating Moles of Mg

The molar mass of Mg is 24.31 g/mol. We can calculate the number of moles of Mg as follows:

${ n_{Mg} = \frac{m_{Mg}}{M_{Mg}} = \frac{3.0 \text{ g}}{24.31 \text{ g/mol}} = 0.123 \text{ mol} }$

Calculating Moles of HCl

The molar mass of HCl is 36.46 g/mol. We can calculate the number of moles of HCl as follows:

${ n_{HCl} = \frac{m_{HCl}}{M_{HCl}} }$

However, we are not given the mass of HCl. We are only given the mass of Mg. To determine the limiting reactant, we need to calculate the mass of HCl required to react with 3.0 grams of Mg.

Calculating Mass of HCl Required

From the equation, we can see that 1 mole of Mg reacts with 2 moles of HCl. Therefore, the number of moles of HCl required to react with 0.123 mol of Mg is:

${ n_{HCl} = 2 \times n_{Mg} = 2 \times 0.123 \text{ mol} = 0.246 \text{ mol} }$

The molar mass of HCl is 36.46 g/mol. We can calculate the mass of HCl required as follows:

${ m_{HCl} = n_{HCl} \times M_{HCl} = 0.246 \text{ mol} \times 36.46 \text{ g/mol} = 8.94 \text{ g} }$

Determining the Limiting Reactant

We have calculated the mass of HCl required to react with 3.0 grams of Mg. However, we are not given the mass of HCl available. To determine the limiting reactant, we need to compare the mass of HCl required to the mass of HCl available.

Scenario 1: Excess HCl

If the mass of HCl available is greater than 8.94 grams, then HCl is in excess, and Mg is the limiting reactant.

Scenario 2: Excess Mg

If the mass of Mg available is greater than 3.0 grams, then Mg is in excess, and HCl is the limiting reactant.

Scenario 3: Equal Amounts

If the mass of Mg available is equal to 3.0 grams, and the mass of HCl available is equal to 8.94 grams, then neither reactant is in excess, and both reactants are limiting.

Conclusion

In conclusion, to determine the limiting reactant in each experiment, we need to calculate the mass of HCl required to react with 3.0 grams of Mg, and compare it to the mass of HCl available. If the mass of HCl available is greater than 8.94 grams, then HCl is in excess, and Mg is the limiting reactant. If the mass of Mg available is greater than 3.0 grams, then Mg is in excess, and HCl is the limiting reactant. If the mass of Mg available is equal to 3.0 grams, and the mass of HCl available is equal to 8.94 grams, then neither reactant is in excess, and both reactants are limiting.

References

Chemistry: An Atoms First Approach, by Steven S. Zumdahl
General Chemistry: Principles and Modern Applications, by Linus Pauling

Additional Resources

Khan Academy: Chemical Reactions
Chemistry LibreTexts: Chemical Reactions

Final Answer

Q: What is the reaction between magnesium and hydrochloric acid?

A: The reaction between magnesium and hydrochloric acid is given by the following equation:

${ Mg_{(s)} + 2 HCl_{(aq)} \rightarrow MgCl_{2(aq)} + H_{2(g)} }$

This equation tells us that when magnesium reacts with hydrochloric acid, it produces magnesium chloride (MgCl2) and hydrogen gas (H2).

Q: What is the limiting reactant in the reaction between magnesium and hydrochloric acid?

A: To determine the limiting reactant, we need to calculate the number of moles of Mg and HCl in each experiment. We can then compare the mole ratio of Mg to HCl to determine which reactant is limiting.

Q: How do I calculate the number of moles of Mg and HCl?

A: To calculate the number of moles of Mg, we can use the following formula:

${ n_{Mg} = \frac{m_{Mg}}{M_{Mg}} }$

Where n is the number of moles, m is the mass of Mg, and M is the molar mass of Mg.

To calculate the number of moles of HCl, we can use the following formula:

${ n_{HCl} = \frac{m_{HCl}}{M_{HCl}} }$

Where n is the number of moles, m is the mass of HCl, and M is the molar mass of HCl.

Q: How do I determine the limiting reactant?

A: To determine the limiting reactant, we need to compare the mole ratio of Mg to HCl. If the mole ratio of Mg to HCl is greater than 1:2, then Mg is the limiting reactant. If the mole ratio of Mg to HCl is less than 1:2, then HCl is the limiting reactant.

Q: What is the significance of the limiting reactant?

A: The limiting reactant is the reactant that is consumed first in a chemical reaction. It determines the amount of product that can be formed.

Q: How do I calculate the mass of HCl required to react with a given amount of Mg?

A: To calculate the mass of HCl required to react with a given amount of Mg, we can use the following formula:

${ m_{HCl} = n_{HCl} \times M_{HCl} }$

Where m is the mass of HCl, n is the number of moles of HCl, and M is the molar mass of HCl.

Q: What is the relationship between the mass of Mg and the mass of HCl?

A: The mass of Mg and the mass of HCl are related by the following equation:

${ m_{Mg} = \frac{m_{HCl}}{2} }$

This equation tells us that the mass of Mg is half the mass of HCl.

Q: How do I determine the amount of product formed in a chemical reaction?

A: To determine the amount of product formed in a chemical reaction, we need to calculate the number of moles of product formed. We can then use the molar mass of the product to calculate the mass of product formed.

Q: What is the significance of the molar mass of a substance?

A: The molar mass of a substance is the mass of one mole of that substance. It is used to calculate the number of moles of a substance and the mass of a substance.

Q: How do I calculate the number of moles of a substance?

A: To calculate the number of moles of a substance, we can use the following formula:

${ n = \frac{m}{M} }$

Where n is the number of moles, m is the mass of the substance, and M is the molar mass of the substance.

Q: What is the relationship between the number of moles of a substance and the mass of the substance?

A: The number of moles of a substance and the mass of the substance are related by the following equation:

${ n = \frac{m}{M} }$

This equation tells us that the number of moles of a substance is equal to the mass of the substance divided by the molar mass of the substance.

Q: How do I calculate the mass of a substance?

A: To calculate the mass of a substance, we can use the following formula:

${ m = n \times M }$

Where m is the mass of the substance, n is the number of moles of the substance, and M is the molar mass of the substance.

Q: What is the significance of the mole ratio of two substances?

A: The mole ratio of two substances is the ratio of the number of moles of one substance to the number of moles of the other substance. It is used to determine the limiting reactant in a chemical reaction.

Q: How do I determine the mole ratio of two substances?

A: To determine the mole ratio of two substances, we can use the following formula:

${ \text{Mole ratio} = \frac{n_{1}}{n_{2}} }$

Where n1 is the number of moles of the first substance and n2 is the number of moles of the second substance.

Q: What is the significance of the mole ratio in a chemical reaction?

A: The mole ratio of two substances in a chemical reaction determines the limiting reactant and the amount of product formed.

Q: How do I calculate the amount of product formed in a chemical reaction?

A: To calculate the amount of product formed in a chemical reaction, we need to calculate the number of moles of product formed. We can then use the molar mass of the product to calculate the mass of product formed.

Q: What is the relationship between the number of moles of a substance and the mass of the substance?

A: The number of moles of a substance and the mass of the substance are related by the following equation:

${ n = \frac{m}{M} }$

This equation tells us that the number of moles of a substance is equal to the mass of the substance divided by the molar mass of the substance.

Q: How do I calculate the mass of a substance?

A: To calculate the mass of a substance, we can use the following formula:

${ m = n \times M }$

Where m is the mass of the substance, n is the number of moles of the substance, and M is the molar mass of the substance.

Q: What is the significance of the molar mass of a substance?

A: The molar mass of a substance is the mass of one mole of that substance. It is used to calculate the number of moles of a substance and the mass of a substance.

Q: How do I determine the amount of product formed in a chemical reaction?

Q: What is the relationship between the number of moles of a substance and the mass of the substance?

A: The number of moles of a substance and the mass of the substance are related by the following equation:

${ n = \frac{m}{M} }$

This equation tells us that the number of moles of a substance is equal to the mass of the substance divided by the molar mass of the substance.

Q: How do I calculate the mass of a substance?

A: To calculate the mass of a substance, we can use the following formula:

${ m = n \times M }$

Where m is the mass of the substance, n is the number of moles of the substance, and M is the molar mass of the substance.

Q: What is the significance of the mole ratio of two substances?

Q: How do I determine the mole ratio of two substances?

A: To determine the mole ratio of two substances, we can use the following formula:

${ \text{Mole ratio} = \frac{n_{1}}{n_{2}} }$

Where n1 is the number of moles of the first substance and n2 is the number of moles of the second substance.

Q: What is the significance of the mole ratio in a chemical reaction?

A: The mole ratio of two substances in a chemical reaction determines the limiting reactant and the amount of product formed.

Q: How do I calculate the amount of product formed in a chemical reaction?

Q: What is the relationship between the number of moles of a substance and the mass of the substance?

A: The number of moles of a substance and the mass of the substance are related by the following equation: