Gen. Chem dilution problem from kaplan prac test

[nicz888]

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Conc H2SO4 is 18M, what volume of H2O would be needed to dilute 25 mL of conc H2SO4 sol'n to 3M

I did this

{25*18}/3 = 150mL

But the answer is 125mL why

Thanks

 

[tinman831]

Moving to DAT>

 

[ajlst11]

The way I do it:

First, find out how many moles of the concentrated H2SO4 you have in 25ml. So, if you have 18 moles in 1000ml, you put 18/1000= x/25 solve:

1000x= 450...........x=0.45 moles in 25 ml. Now, you need 3M, so:

0.45/x= 3/1000.............3x=450.........x=150, so you need a TOTAL of 150ml. You started with 25ml, so the answer is Total-Start = 150ml - 25ml = 125ml

Hope it helps

 

[ajlst11]

Forgot to tell you this simple equation:
(M1)(V1)=(M2)(V2)

(18M)(0.25L) = (3M)(X)

0.450= 3X

X=0.150L, so 150mL is V2........150-25=125 There ya go

This is a VERY important equation, remember it. Any more questions, just throw them my way. Glad to help