1.1.7: How to Solve Chemistry Problems
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!
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: [latex]\mathrm{density=\dfrac{mass}{volume}}[/latex] [latex]\mathrm{d=\dfrac{m}{V}}[/latex]
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
[latex]1.15\dfrac{g}{cm^3}x\dfrac{1 cm^3}{1 mL}[/latex] = 1.15 g/mL
calculate mass in grams
[latex]1.15\dfrac{g}{mL}x(32.00 mL)[/latex] = 36.8 g
convert mass to mg
[latex]36.8 g(\dfrac{1000 mg}{1 g}) = 36800 mg[/latex]
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
[latex]36800 mg = 3.68 x 10^4 mg[/latex]
Check – does it make chemical sense? Yes, 36.8 g > 32 g which makes sense based on the density value given.