# MCAT Physical : Reaction Calculations and Limiting Reagent

## Example Questions

### Example Question #41 : Stoichiometry And Analytical Chemistry

5.6 grams of manganese reacts with 650 mL of 6.0 M hydrochloric acid to form manganese (V) chloride and hydrogen gas. Along with the products, a large amount of heat is evolved.

Assuming standard temperature and pressure, what volume of hydrogen gas is produced by this reaction?

Explanation:

Convert 5.6g of manganese (limiting reagent) to volume of hydrogen gas.

### Example Question #11 : Reaction Calculations And Limiting Reagent

Balance the chemical reaction.

Explanation:

The reaction given is the complete oxidation of glucose. Reactants are glucose and oxygen (from inhalation) and products are carbon dioxide and water, which are released through exhalation and excretion.

The complete balanced reaction is:

When balancing reactions, it is generally easiest to leave oxygen and hydrogen alone until the end. First, balance the carbon. Next, balance the hydrogen since it is only found in one reactant molecule and one product molecule. Finally, balance the oxygen.

### Example Question #11 : Reaction Calculations And Limiting Reagent

Given the unbalanced equation below, how many grams of carbon dioxide will be produced from one mole of glucose and three moles of oxygen?

Explanation:

The first step to solve will be to balance the chemical reaction:

We see that we see that for every one mole of glucose used, six moles of carbon dioxide will be made. Similarly, for every six moles of oxygen used, six moles of carbon dioxide will be formed. For the reaction to carry out to completion, however, there must exist six moles of oxygen for every one mole of glucose. In the problem's circumstances, one of these compounds becomes the limiting reactant, in this case it is oxygen.

We only have three moles of oxygen, but we would need six to react all the given glucose, making oxygen the limiting reagent. We need to find the carbon dioxide produced from the limited amount of oxygen present. Use the molar ratio between oxygen and carbon dioxide and the molar mass of carbon dioxide to solve.

### Example Question #11 : Reaction Calculations And Limiting Reagent

Five kilograms of oxygen are consumed in a chemical reaction that generates two photons per oxygen molecule. How many photons were generated?

Explanation:

First, use Avogadro's law and the molar weight of oxygen to determine the number of oxygen atoms.

We know that two photons are formed from every oxygen molecule. We can use basic stoichiometry to find the number of photons generated.

### Example Question #11 : Reaction Calculations And Limiting Reagent

Given the unbalanced equation above, how many moles of hydrocholoric acid would be required to produce four moles of potassium chloride?

32

4

64

16

8

32

Explanation:

It is first necessary to balance the equation.

So, sixteen moles of hydrochloric acid (HCl) would produce two moles of potassium chloride (KCl). Multiplying this ratio times two, thirty-two moles of HCl would produce four moles of KCl.

### Example Question #11 : Reaction Calculations And Limiting Reagent

A chemist combines 100g of zinc and 200g of copper (II) sulfate. According to the above chemical reaction, how many grams of zinc sulfate are formed?

303g

101g

494g

247g

202g

202g

Explanation:

We don't know which of the reactants is limiting, so we'll need to calculate how much zinc sulfate would be produced by the given amount of each of the reactants, assuming the other is in excess. Whichever reactant would produce less zinc sulfate is the limiting reactant, and the amount of zinc sulfate produced by that reactant is the actual amount that can be produced by this mixture.

If we start with 100g of zinc, we yield 247g of zinc sulfate.

Starting with 200g of copper(II)sulfate, we yield 202g of zinc sulfate.

Less zinc sulfate can be produced by the given amount of copper(II) sulfate, so copper(II) sulfate is the limiting reactant, and 202g of zinc sulfate are actually produced by the given mixture.

### Example Question #11 : Reaction Calculations And Limiting Reagent

Sodium chloride can be created by the following reaction.

What type of reaction is shown here?

Single-replacement

Combination

Decomposition

Double-replacement

Single-replacement

Explanation:

In this reaction, only one element is replaced by another. NaBr becomes NaCl, with a single-replacement of chlorine for bromine. The diatomic halogens are not considered as replacing one another, as they are not bound to a cation.

A comparison of single- and double-replacements reactions is shown below.

Single-replacement:

Double-replacement:

### Example Question #12 : Reaction Calculations And Limiting Reagent

The combustion of liquid hexane in air at 298K gives gaseous carbon dioxide and liquid water. Write a balanced chemical equation for this reaction, including the physical states of all the compounds involved.

Explanation:

Any combustion reaction of a hydrocarbon involves oxygen gas as a reactant, and produces carbon dioxide and water as products. In this case, two moles of hexane react with nineteen moles of oxygen to produce twelve moles of carbon dioxide and fourteen moles of water. Carbon dioxide is a gas at room temperature, and water is a liquid.

Hexane:

Reaction:

Now we can begin to balance the reaction.

Everything is balanced except oxygen; there is an odd number of oxygen to the right and an even number to the left. We can adjust this by multiplying everything by two.

### Example Question #12 : Reaction Calculations And Limiting Reagent

Complete the reaction.

Explanation:

The reaction of a salt, like , with an acid, like , is a double-replacement reaction. The products are a salt of the original acid (), and a second acid formed from the original salt (). Remember to keep the equation balanced.

### Example Question #51 : Stoichiometry And Analytical Chemistry

When sodium nitrate reacts with iron (III) bromide to produce iron (III) nitrate, what is the molar ratio of iron (III) nitrate to sodium bromide?

1:2

3:1

1:3

1:1