NaCI of 95% purity is used to prepare salt cake (Na2SO4) by the reaction,

2NaCI + H2SO4 → Na2SO4 + 2HCI

If the product (Na2SO4) is only % pure, what weight of NaCI is used up in producing 1 kg of the impure salt cake?

PLEASE EXPLAIN A LITTLE IN DETAIL!

Since you have not mentioned the % purity of the product
So, if we assume that the Na2SO4 so formed is x% pure.
We can solve the ques as,

If both NaCl and Na2SO4 were 100 % pure, one can say that,
2*58g NaCl gives Na2SO4 = 142 g
if NaCl is regarded as 95 % pure,
one can say, 95*2*58/100 g NaCl gives Na2SO4 = 142 g
i.e, 110 g NaCl gives Na2S04 = 142 g
or vice verca, 142 g is obtained by NaCl = 110 g
if we take into account the percent purity of Na2SO4 also,
142*x/100 g Na2SO4 is obtained by NaCl = 110g
1 Kg Na2SO4 is obtained by NaCl = 110*100*1000/(142*x)

You can replace the value of x to get the required answer.