Posted by Fai on Tuesday, December 25, 2012 at 12:31am.
In scientific calculations, it is important to know how accurate you'd like your answers. If you see a number like 0.9864g/ml, it means 4 significant figures are expected, so you have to carry your calculations to at least 5 significant digits, and then round off to 4 for the final answer.
Except for the number of significant digits, your calculation of molarity is correct.
Molality is defined as the ratio of the number of moles of solute to the mass of solvent.
For this problem, we need to find the mass of solvent (mass of solute is known, 10 g). Since the final volume is known (250 ml), so is the density (0.9864), the mass of the solution is therefore
m(solution) = 250*0.9864=246.60 g
Next calculate 10 g of NaCl is equivalent to how many moles (M), using accurate atomic masses.
The molality is then
M mol /(1000 g/kg /(246.6 g - m g)
The molality is then
M mol /(1000 g/kg /( m -10 g)
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