Question

How many mL of concentrated ammonia (14M) should be diluted to 250mL to make a 0.75 M NH3 solution?

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

Mstock = 14 M, Vstock = X

Mfinal = 0.75 M, Vfinal = 250 mL

14*Vstock = 0.75*250

Vstock =  0.75*250/14

Vstock = 13.39 mL of stock solution required