`a)`
$m_{dd}$=$m_{ct}$+$m_{dm}$`=20+600=620` `g`
`C%_{KCL}=`$\dfrac{m_{ct}}{m_{dd}}$`.100=20/620 .100=3,22%`
`b)`
$\text{Đổi:}$`50` `ml``=0,5` `l`
$m_{H_2SO_4}$=$C_{M}$.$V_{dd}$`=2.0,05=0,1` `mol`
$m_{H_2SO_4}$=$M_{H_2SO_4}$.$n_{H_2SO_4}$`=0,1.98=9,8` `g`