1.1.7: How to Solve Chemistry Problems
- Page ID
- 395084
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)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: \(\mathrm{density=\dfrac{mass}{volume}}\) \(\mathrm{d=\dfrac{m}{V}}\)
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.