Answer: 10 ml of 200 mM [tex]CaCl_2[/tex] is required and 30 ml of water is required.
Explanation:
According to the dilution law,
[tex]C_1V_1=C_2V_2[/tex]
where,
[tex]C_1[/tex] = concentration of stock solution = 200mM
[tex]V_1[/tex] = volume of stock solution = ?
[tex]C_2[/tex] = concentration of resulting solution= 50mM
[tex]V_2[/tex] = volume of another acid solution= 40 ml
[tex]200\times x=50\times 40[/tex]
[tex]x=10ml[/tex]
Thus 10 ml of 200 mM [tex]CaCl_2[/tex] is required and (40-10) ml = 30 ml of water is to be added to make 40 ml of 50mM [tex]CaCl_2[/tex].