Chem Dilutions question - SO AGGRAVATING!

[annieannie]

I am doing some practice questions right now and this is making me so mad because I keep getting these dilution questions wrong!

What volume of water would be needed to dilute 50mL of a 3MHCl solution to 1M?
a. 25ml
b. 50ml
c. 75ml
d. 100ml
e. 150ml

The answer is D. WHY?
Am I missing something here? because I think the answer is E

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[Kahr]

I am doing some practice questions right now and this is making me so mad because I keep getting these dilution questions wrong!

What volume of water would be needed to dilute 50mL of a 3MHCl solution to 1M?
a. 25ml
b. 50ml
c. 75ml
d. 100ml
e. 150ml

The answer is D. WHY?
Am I missing something here? because I think the answer is E

I've been reviewing orgo so much lately (neglecting gen chem) that this confused me a tad too. After doing a little review I think I have come upon your error.

You're using the good old M1V1 = M2V2, right? With this, V2 does equal 150. But what is V2? V2 is the final volume, it's everything, the original volume of acid plus the water, right? So technically the water added has to be less than V2 (the final volume)... so I just subtracted V2 from the original volume of acid to get the water added. (150 - 50) = 100mL water added.

I always find myself wondering if I'm explaining intelligibly, please let me know if this makes sense.

Cheers

 

[tendram]

Kahr is right. To any dilution problem using M1V1=M2V2 you are always going to get the total volume needed to have the desired molarity. You will always have to subtract your initial volume from your final volume when using this method.