Question

A diluted solution of Red Dye #40 is 6.50x10-3 M. If the solution resulted by diluting 5.00 mL of Red Dye #20 to 500.0 mL, what was the original solution concentration?

Answer 1

We need to apply dilution law, which is based on the mass conservation principle

initial mass = final mass

this apply for moles as weel ( if there is no reaction, which is the case )

mol of A initially = mol of A finally

or, for this case

moles of A in stock = moles of A in diluted solution

Recall that

mol of A = Molarity of A * Volume of A

then

moles of A in stock = moles of A in diluted solution

Molarity of A in stock * Volume of A in stock = Molarity of A in diluted solution* Volume of A in diluted solution

Now, substitute known data

M!*V1 = M2*V2

M1 = M2*V2/V1 = (6.5*10^-3)(500)/(5) = 0.65 M