Question

массовая доля Na2SO4

Answer 1

 Mr(Na2SO4)= 23*2+32+16*4= 142

Answer 2

Mr (Na2SO4) = 23*2 + 32 + 64 = 142 г/моль
w (Na) = 46/142*100% = 32,4%
w (S) = 32/142*100% = 22,5%
w (O) = 64/142*100% = 45,1%