Question

How would you make 250 mL of 0.150 M sodium thiosulphate from solid $N{a_2}{S_2}{O_3}.5{H_2}O$(molar mass= 248.2 $gmo{l^{ - 1}}$)

Answer 1

Here the M used in 0.150 M is the molarity. The molarity is defined as the number of moles of the solute which dissolved in per litre of the solution. It is also known as the molar concentration. It depends upon the total volume of the solution. It is expressed as M .

Complete step-by-step answer: The formula for molarity is the number of solute moles divided by the volume of solution which is expressed as the following:
Molarity=$\dfrac{{number\, of \,moles}}{{volume\, of \,solution(L)}}$
So we know that the formula for number of moles is the given mass by the molar mass which is expressed as the following:
Number of moles=$\dfrac{{given \,mass(w)}}{{molar \,mass(M)}}$
We can substitute the formula of the number of moles in the molarity formula. So it will be then expressed as the following:
Molarity=$\dfrac{{given\, mass(w)}}{{molar\, mass(M) \times volume\, solution(L)}}$
So the molarity is 0.150M
The volume of solution is 250 mL which is to be in litre which will be equal to 0.25L
The molar mass of the sodium thiosulphate is 248.2 $gmo{l^{ - 1}}$
Now substituting the values in formula we get,
0.125=$\dfrac{{given mass(w)}}{{248.2 \times 0.25}}$
On further simplifying we get,
Given mass = 7.756g of the sodium thiosulphate.

Note: The molarity of the solution depends on the physical properties of the system such as the pressure or the temperature. The one molar solution is the solution in which 1 g of solute is dissolved in 1 litre of the solution. In molarity we take the full volume of solution because both solute and solvent blend the solution.