Solving chemistry word problems

[john16O]

when is comes to theory in chemistry I have a good understanding. But when it comes to word problems such as molality, %mass, dimensional analysis, molarity, etc. i tend to get these question wrong. I do not know if it is because I am not a very good problem solver but I cannot seem to get these types of questions right on the test. and then when I go to my professor and she explains to me how to solve one, i say to myself, 'why didn't i think of that?', but i never have an answer to that question. So i was just wondering if someone could offer up a couple steps that they go through when solving these kinds of problems. or recommendations on how I could improve my skills in this area. Thank you to everyone for your help in advance!

 

[mgb_phys]

Have an estimate of what the answer should be before you start putting numbers into a calculator. If the atomic mass of Sodium is 20 something and chlorine is 30something then 0.1mol of sodium chloride better come out to a few grams, if you get an answer like 30kg you probably have somethign upside down.

Check dimensions. If a number is given in g/mol and you want an answer in moles then the g/mol number must be on the bottom of the equation.

Check units - masses are given in grams but everything else is in meters/kilograms. Especially with gas laws check that you have grams / kg consistently.

 

[john16O]

ah i see, but how would I approach a problem like this one:

commercial aqueous nitric acid has a density of 1.42 g/mL and is 16 M. Calculate the molality of this solution.

-I know what molality is: moles of solute/kg of solvent. but how do i manipulate these unites to get them into that format? you know what i mean?

 

[mgb_phys]

I would assume they want moles/litre.
You know the formula of Nitric Acid so you can work out the mass, you then know the moles/g, you have the density so can work out the moles/litre.

 

[john16O]

ah! i see thank you!