[zerality]

Hi,
I am getting confused as how to calculate molarity in a lab experiment performed in class.
We had to determine the concentration of solids, reported as NaCl, in a sample of seawater by:
 
determine density and mass of a sample of seawater. we boiled the seawater to dryness and measured the mass of the residual solids. from the data we had to calculate the % by mass, M, and m of the seawater sample.

i managed to calculate % by mass (3.12%) and the molality, but am confused as how to calculate M.
to calculate m  i assumed a 1000g solution which would then be composed of 31.2 grams of NaCl and 1000-31.2 grams (968.8 g) H2O. Therefore molality would equal 31g/.9688 kg  = 32 g NaCl/Kg H20 = 32/58.44 = .55 m

To calculate molarity: I said density = mass/voulme; mass (solution) = 1.020g/ml x 1000ml = 1020 g
since % by mass of NaCl is 3.12%, in 1000ml the mass = (1020 x 3.12)/(100) = 31.82g NaCl.
therefore the moles of NaCl in 1 liter of solution is 31.82/58.44 = .54   M= .54
Is my answer for M correct?

Many Thanks

[sdekivit]

you know the density of the solution: 1,02 g / mL and 3,12 mass% = NaCl

Now it's easier to take 100 g seawater, since percetage means: per hundred.

know 100g seawater thus contains 3,12 g NaCl and using the density, you can calculate the volume of 100g seawater.

Then you need to reconsider the definition of molarity: the amount of mol solute per L solvent.

[zerality]

Thanks for the response.

Isn' the definition of molarity:  moles solute/liter of solution. If so, isn't my answer correct?

[Yggdrasil]

Molarity is defined as moles of solute per liter of solution

http://en.wikipedia.org/wiki/Molarity#Molarity

[Borek]

http://www.chembuddy.com/?left=concentration&right=molarity

[zerality]

So is my answer correct?

[Yggdrasil]

It's right if you don't need to worry about significant figures.