If the concentration of NaCl is 0.25M, calculate
a.how many moles of NaCl there are in 20mL of the solution
0.005mol
b.what volume of the NaCl must be taken to contain 0.1mol of NaCl.
0.4L

Are these are correct?

Thnk you :)

1. The concentration of NaCl is 0.25 molar. That's 0.25 moles per liter. You have 20 mL of that solution

0.25\,\frac {moles}{\cancel {Liter}}\,\times\, 20\,\cancel {mL} \,\times\, \frac {1\,\cancel {Liter}}{1000\,\cancel {mL}}= \frac {0.25\,\times\,20}{1000}\,moles = 0.005\, moles

Yup. Looks right.

2.
0.25 \,\frac {moles}{Liter}\,V = 0.1\,mol

V = 0.1 \cancel {mol} \times \frac {Liter}{0.25 \, \cancel {mol}} = 0.4\,Liter

Yup. That looks right, too.

Congratulations.