Question

solution of sodium thiosulfate of volume V=50ml and molar concentration C=0.1mol/l is prepared by dissolving a mass(m) of solid penta-hydrated sodium thiosulfate Na2S2.5H2O in water.
Calculate the mass(m).

Answer 1

solution of sodium thiosulfate of volume V=50ml and molar concentration C=0.1mol/l is prepared by dissolving a mass(m) of solid penta-hydrated sodium thiosulfate Na2S2.5H2O in water.

Calculate the mass(m).

Solution:

Firstly, we calculate the molar mass of Na₂S₂O₃ and Na₂S₂O₃·5H₂O:

M(Na₂S₂O₃)= 23·2+32·2+16·3=158 g/mol;

M(Na₂S₂O₃·5H₂O)= M(Na₂S₂O₃)+5M(H₂O)=158+5·(16+1·2)=248g/mol.

So, we can determine how much Na₂S₂O₃·5H₂O heavier than (Na₂S₂O₃):

$$\frac {M \left(N a _ {2} S _ {2} O _ {3} \cdot 5 H _ {2} O\right)}{M \left(N a _ {2} S _ {2} O _ {3}\right)} = \frac {2 4 8}{1 5 8} = 1. 5 6 9$$

Then, we calculate the mass of Na₂S₂O₃ in solution:

m(Na₂S₂O₃)=C[mol/l]·M(Na₂S₂O₃)[g/mol]·V[l]=0.1·158·0.05=0.79 g.

Finally, we calculate the mass of Na₂S₂O₃·5H₂O:

m(Na₂S₂O₃·5H₂O)= m(Na₂S₂O₃) · 1.569=0.79·1.569=1.24 g.

Answer: m(Na₂S₂O₃·5H₂O)=1.24 g.