stoichiometric factor

ratio of coefficients in a balanced chemical equation, used in computations relating amounts of reactants and products

relationships between the amounts of reactants and products of a chemical reaction

Chemistry 2e · 4.3 Reaction Stoichiometry

Stoichiometry practice problems

7 problems from the book, with its answers. Check your working, or have any of them explained to you on the whiteboard.

How to approach these problems

Every problem here is the same three steps. Turn what you are given into moles: mass ÷ molar mass, or volume × molarity for a solution. Use the coefficients of the balanced equation as a mole ratio to get from the given substance to the one asked for. Then turn moles back into what the question wants: mass, molecules (× 6.022 × 10²³) or volume. Write units on every number and let them cancel; if they don't, the ratio is upside down. The usual mistakes: an unbalanced equation, using a mass ratio instead of the mole ratio, and rounding in the middle. Keep extra digits until the end, then round to the least precise measurement.

Exercise 47, worked through on the whiteboard

A real lesson: the tutor solves it step by step on the board, the way it would with you.

Read what the tutor says

Hey Seo Pilot, I'm Theo! Let's break down this stoichiometry problem together.

We're starting with a solution volume and molarity of hydrochloric acid, and we need the moles and mass of gallium chloride produced.

Looking at part A, how would you find the moles of hydrochloric acid from that volume and concentration?

Boom! Perfect calculation.

Litres cancel neatly, leaving us with moles of HCl.

Now for step two: looking at the coefficients in our balanced reaction, what mole ratio converts HCl to GaCl3?

Nice work! That simplifies to one third, landing right at one point two five moles rounded.

Now the question also asked for the mass of GaCl3. What's the molar mass of GaCl3?

Take a quick look at the periodic table values on the right: what do gallium and those three chlorines add up to?

Spot on! One hundred seventy-six point zero seven grams per mole.

Now for the final stretch: how do we turn our moles of GaCl3 into grams?

What do you get when you multiply those out, keeping significant figures in mind?

Boom, right on the money! Two point two times ten to the second grams matches the book perfectly.

You nailed every single calculation here! What did your original attempt look like that was giving you trouble?

Exercises 47
Gallium chloride is formed by the reaction of 2.6 L of a 1.44 M solution of HCl according to the following equation: $2 Ga + 6 HCl ⟶ 2 GaCl_{3} + 3 H_{2} .$
(a) Outline the steps necessary to determine the number of moles and mass of gallium chloride.
(b) Perform the calculations outlined.
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Exercises 49
Silver is often extracted from ores such as K[Ag(CN)₂] and then recovered by the reaction
$2 K [Ag (CN)_{2}] (a q) + Zn (s) ⟶ 2Ag (s) + Zn (CN)_{2} (a q) + 2 KCN (a q)$
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Exercises 51
Carborundum is silicon carbide, SiC, a very hard material used as an abrasive on sandpaper and in other applications. It is prepared by the reaction of pure sand, SiO₂, with carbon at high temperature. Carbon monoxide, CO, is the other product of this reaction. Write the balanced equation for the reaction, and calculate how much SiO₂ is required to produce 3.00 kg of SiC.
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Exercises 53
Urea, CO(NH₂)₂, is manufactured on a large scale for use in producing urea-formaldehyde plastics and as a fertilizer. What is the maximum mass of urea that can be manufactured from the CO₂ produced by combustion of $1.00 \times 10^{3} kg$ of carbon followed by the reaction?
$CO_{2} (g) + 2 NH_{3} (g) ⟶ CO (NH_{2})_{2} (s) + H_{2} O (l)$
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Exercises 55
A compact car gets 37.5 miles per gallon on the highway. If gasoline contains 84.2% carbon by mass and has a density of 0.8205 g/mL, determine the mass of carbon dioxide produced during a 500-mile trip (3.785 liters per gallon).
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Exercises 57
What volume of a 0.2089 M KI solution contains enough KI to react exactly with the Cu(NO₃)₂ in 43.88 mL of a 0.3842 M solution of Cu(NO₃)₂?
$2 Cu (NO_{3})_{2} + 4 KI ⟶ 2 CuI + I_{2} + 4 KNO_{3}$
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Exercises 59
The toxic pigment called white lead, Pb₃(OH)₂(CO₃)₂, has been replaced in white paints by rutile, TiO₂. How much rutile (g) can be prepared from 379 g of an ore that contains 88.3% ilmenite (FeTiO₃) by mass?
$2 FeTiO_{3} + 4 HCl + Cl_{2} ⟶ 2 FeCl_{3} + 2 TiO_{2} + 2 H_{2} O$
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Key terms · 2

stoichiometric factor: ratio of coefficients in a balanced chemical equation, used in computations relating amounts of reactants and products
stoichiometry: relationships between the amounts of reactants and products of a chemical reaction

Learn 4.3 on the whiteboard

The tutor teaches the whole section, drawing it out step by step and answering your questions as you go.

Read 4.3 in the textbook · This section on openstax.org