[Rutherford]

For the purpose of obtaining NaCl, a previously calculated mass of Na₂CO₃ was added to 5% solution of HCl. What was the mass share of NaCl in the obtained solution?

I thought this way:
xNa₂CO₃+2xHCl 2xNaCl+xH₂O+xCO₂
Mass of HCl taht reacted was 71x, so the mass of water in the 5% solution should be 1349x. 18x of water is produced, and 44x of CO₂ is released and 117x of NaCl was produced, so: ω=117x/(117x+1349x+18x-44x)*100=8.125%, but the correct answer is 7.7%. Where am I wrong?

[Borek]

Looks to me like your answer is the correct one, and they forgot to account for the fact only 95% of the acid solution is water.

[Rutherford]

Yes, when I put now the 71x from HCl in the equation I get the answer, but it shouldn't be put, so their answer is wrong. Thanks for making this clear.