how to calculate the molality?

Question

dissolve 1.0gNaCl in 100g(ml)in water and dissolve 2.0g NaCl in 100g(ml) in water.how to calculate molality of the two solutions?

Answer 1

Go to ----> http://www.epa.gov/eogapti1/module2/rate/rate.htm <---. This site has the formulas you need to complete the equation. I would solve it for you, but you need to figure it out on your own to get the benefits of learning. cheers!

Answer 2

To calculate molar concentration, c, we use:

c = n / V, where:
n = number of moles of solute (NaCl)
V = volume of solvent (water)

So, we need to get n. We can use:

m = n * M, where:
m = mass we have of the substance
M = molar mass of substance (the mass of one mole of the substance)

We can rearrange this to get:

n = m/M

We have m, but we need to calculate M. For a molecular substance, we use the molecular formula to calculate M. In this case, we have an ionic substance (NaCl), not composed of molecules, so we use the empirical formula. We simply add up the molar masses of each of the components to calculate the molar mass of the substance:

M(NaCl) = M(Na) + M(Cl) = 22.990g + 35.453g = 58.443g

[ Note that the coefficients of each atom in the substance's formula must be taken into account, so for a molecular substance such as C2H6, we would calculate: M(C2H6) = 2 * M(C) + 6 * M(H) ]

Now we can calculate n, using M = 58.343g and m = 1.0g, which was already given:

n = 1.0g / 58.343g = 0.0171 mol

This n = 0.0171 mol and V = 100mL = 0.1L can be substituted into the equation c = n / v to give:

c = 0.0171mol / 0.1L = 0.171 mol/L = 0.171M
(Note that M here is the symbol for molarity, or moles per litre, rather than representing molar mass as in the equation m = n*M).

The method to work out the answer for the 2.0g NaCl part is the same, so try it yourself. Also, you may want to check over my working, etc, in case I made a typo.

Seen as you're asking this, I think you've probably already been given the formulae you need, but you're still stuck.

Anyway, so you can do this in an exam, you need to go practice more questions like this. I'm sure your teacher will have given you or will be giving you more questions like this. Don't worry about the length of my answer. It only looks long because I did every little step with an explanation. When you learn how to do these questions, they'll only take you a couple of lines to work out. But, you should write out the formulae you use each time you do a question. That's how I used to memorise formulae when I was in high school.