1.1.7: How to Solve Chemistry Problems

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In CHEM 1510 and 1520 we spend a lot of time solving qualitative and quantitative problems in chemistry. It is helpful to have a consistent approach to solving problems.

Read the question carefully and highlight what information you are given and what information you are asked to find.
Make a list of what information you have and identify which items you need to use. Make a list of what you need to find.
If this is a quantitative problem look carefully at the units of the information you have and the units of what you need to find. The units will tell you what to do!
Plan your approach to solve the problem. Make a prediction about the answer using your chemistry knowledge.
Identify the equation or concept you need to use to solve the problem. If this is a quantitative problem perform any unit conversions needed.
Solve the problem. If this is a quantitative problem make sure you follow significant figures through the problem and express final answer with correct significant figures and units.
Check your answer. Compare your result with your prediction and ask yourself if it makes chemical sense.

What do I KNOW?

What do I WANT?

HOW can I solve for it?

PREDICT?

SOLVE the problem!

CHECK your answer!

Example \(\PageIndex{1}\)

An aqueous solution has a density of 1.15 g/cm³. How much would 32.00 mL of this solution weigh in mg?

Solution

What do I KNOW? Given: density solution 1.15 g/cm³; volume solution 32.00 mL

What do I WANT? Need to find: mass in mg

HOW can I solve for it? Look at units and plan:

volume in mL, need density to have same units then can solve for mass in g, and convert to mg

equation to use: \(\mathrm{density=\dfrac{mass}{volume}}\) \(\mathrm{d=\dfrac{m}{V}}\)

Prediction: mass will be over 32 g (and less than 40g) since density just over 1

Solve:

convert density to g/mL

recall - conversion factors have infinite sig figs

\(1.15\dfrac{g}{cm^3}x\dfrac{1 cm^3}{1 mL}\) = 1.15 g/mL

calculate mass in grams

\(1.15\dfrac{g}{mL}x(32.00 mL)\) = 36.8 g

convert mass to mg

\(36.8 g(\dfrac{1000 mg}{1 g}) = 36800 mg\)

express with correct significant figures and units

Need 3 sig figs in final answer since density given with 3 sig figs and multiplying & dividing, so use scienfitic notation

\(36800 mg = 3.68 x 10^4 mg\)

Check - does it make chemical sense? Yes, 36.8 g > 32 g which makes sense based on the density value given.