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/cm 3 . How much would 32.00 mL of this solution weigh in mg?
Solution
What do I KNOW ? Given : density solution 1.15 g/cm 3 ; 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.