We have to know this:
Solution here = HCl (solute) + Water (solvent)
Solution 1)
Volume 1 (V1)= 50 mL
Concentration 1 (M1) = 0.2 M
We add 200 mL of water into Solution 1, so we will have a diluted solution which is Solution 2.
Solution 2)
Volume 2 (V2, be careful with the dilution) = 50 mL + 200 mL (added) = 250 mL
Concentration 2 (M2) = Need to be found
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The formula we use to calculate concentration 2 is:
V1 x M1 = V2 x M2, we clear M2 from here and we get to:
M2 = (V1 x M1) / V2 = (50 mL x 0.2 M) / 250 mL = 0.04 M
M2 = 0.04 M ( we can say that it is right because when we add water, concentration of the solution will decrease)
Answer: M2 = 0.04 M (mol/L)