The amount of 0.2 M stock NaCl that would be required to make 100 mL, 0.05 M NaCl would be 25 mL
Using the equation of dilution:
M1V1 = M2V2
In this case, M1 = 0.2M, V1 = ?, M2 = 0.05 M, V2 = 100 mL
V1 = M2V2/M1
= 0.05 x 100/0.2
= 25 mL
The volume of the stock solution needed is 25 mL
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