Molar concentration? 10 points WILL be rewarded!?

Question

Calculate the molar concentration of a solution made by dissolving 8.0 grams of solid NaOH in 500 mL of solution.

Calculate the molar concentration of a solution made by diluting 8.75 mL of 12.4 M hydrochloric acid to 1.00 L of solution.

Answer 1

Google is your best friend.

Answer 2

NaOH

23+16+1=40

No of moles=8/40

                 =.2 moles

.2 moles/500 ml

.4 moles/dm3

Answer 3

top:

8g of NaOH makes 0.2 moles

0.2 moles in 0.5l makes it 0.4M (moles per litre = moles per decimetre which is standard unit)

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bottom:

0.00875 L of 12.4M = 0.1085 moles

0.1085 moles in a litre makes it 0.1085M

is this GSCE homework or something? (btw i'm not putting money on the fact I've not made a mistake... it's the summer and so my chem is rusty.)

Answer 4

The way you do the first kind of problem is to first realize that you need your answer in moles/liter.  Know the units you need, cancel the units you don't and you can solve a lot of problems where you don't remember (or never even knew) the way the teacher wanted you to do things.  For example.  I need moles per liter...have grams...have liters.  Well, I need liters in denominator so I will divide by liters at some point.  To get moles from grams I use molecular weight of NaOH which I remember is in grams per mole. Mass in Grams/(molecular weight in grams/mole) gives moles.  Moles/volume in liters gives concentration.  I could also do things another way:

mass in grams/(volume in liters*molecular weight in g/mol)  still gives me mol/liter.

You just have to make the units cancel.

In dilution, remember you never change the number of moles of the compound you are diluting.  Moles = concentration*volume so:

starting concentration*starting volume = final concentration*final volume

That last equation is your friend.  You use it A LOT in chemistry.