Solving Quantitative Problems
H/Chemistry
Hart
Much of your work in chemistry will focus on solving quantitative problems. Many such
problems will be word problems. Learning to solve a variety of problems with ease requires
practice. The steps outlined below will help you analyze and solve even complicated word
problems correctly.
Four Steps for Solving Quantitative Problems
Step 1: Analyze
Read the problem carefully at least twice.
Note any important descriptive terms that add meaning to the problem.
e.g. Are you dealing with solutions or mixtures? Is a reaction occurring?
List all the data given in the problem.
Next, identify what you are asked to find including the units.
Many people find it helpful to sketch what they think is happening in the problem. Making a
sketch labeled with the data and what you are asked to find shows you truly understand the
problem.
Step 2: Plan
Your plan for solving a problem should show how the information given in the problem is
used to find a solution.
Reread the problem to note if there is any information missing from the problem that you will
need to find the answer.
To finish your plan, decide which conversion factors or algebraic equations relate to what you
are asked to find.
Your plan might consist of a single calculation using one conversion factor or a series of
calculations and conversion factors.
The factor-label method (~ dimensional analysis) is a very useful tool for verifying the setup.
If the units cancel to give the desired units, the setup is probably correct.
Step 3: Compute
This step involves substituting data and conversion factors into your plan from Step 2.
Calculate the answer, cancel the units, and round off to the correct number of significant figures.
Step 4: Evaluate
Always look back at your answer to determine if it is reasonable.
There are various ways to verify your answer.
1.
Make an estimate of the answer. Is your calculated value close to the estimate?
2.
Does the setup provide the correct units for the answer?
3.
Does the answer make sense from what you know about values for the problem?
e.g. If you calculate the velocity of a car to be 5500 km/h, you know this answer is not
reasonable. A car traveling at the maximum speed on a highway is only moving about
100 km/h.
from Modern Chemistry, Tzimopoulos et al., 1993