8.58g of coastic soada was dissolved to produce a solution of 500cm3 .to neutralize 50cm3 of this solution 15cm3 of 0.4M HCl was consumed .(coastic soada = Na2CO3.XH2O ) find the value of X?

Answer 1

Note the correct spelling of caustic soda. Also, correct sentence structure helps us read these problems better.
How many mols HCl did you use? That's M x L = 0.4M x 0.015L = 0.006.
How many grams were in the solution you titrated. That's 8.58 x 50/500 = 0.858 g.
Then mols = grams/molar mass and rearrange to molar mass = grams/mols.
0.858/0.006 = about 143.
Subtract Na2CO3 = -106
Remainder is 37 and 37/18 = 2.05 mols and I would round that to 2.0 for X.