[pre med UK]

Hi guys i have this question, i've spent about 6 hours trying to decide how the answer was worked out!! i would be super grateful if you could explain this, since its making me very upset since its a relatively easy subject

Question
You are asked to make up 60ml of 0.08 M Mg2+ in solution, unsure of this you add 10 ml of 0.009 M MgCL2 and 50 ml of Mg2(PO4)2.
What conc of Mg2(PO4)2 is required to achieve the 0.08 M solution?
the answer is 0.047,

25 ml of 0.150 K2CO3 is added to a beaker holding 75ml of a 0.400 potassium ion solution. The new potassium ion concentration is 0.037
How did you arrive to that answer??

Please help me since it is stopping me revise other subject areas, which i need to do for an entrance exam into medical school. Thanks

[sjb]

Question
You are asked to make up 60ml of 0.08 M Mg2+ in solution, unsure of this you add 10 ml of 0.009 M MgCL2 and 50 ml of Mg2(PO4)2.
What conc of Mg2(PO4)2 is required to achieve the 0.08 M solution?
the answer is 0.047

0.047 what? kilograms per pint?

Anyway 60ml of 0.08M Mg²⁺ is what amount of Mg²⁺?

You've added some from the chloride, how much?

So you need to add how much from the phosphate (BTW, are you sure about that formula, the charges don't seem to balance to me, but that's possibly irrelevant)?

As you have a volume, and an amount for the phosphate you can then calculate a concentration.

S

[pre med UK]

thats exactly how the question was written, there were no other data give, so i assume its one has to work out the question from the information in the question!?

the 0.047 (the answer) refers to the concentration of the Mg2(PO4)2 required to make the 0.08 concentration of 60 mls

i'm don't know how you arrived at that ? pls could you demonstrate?

[sjb]

Anyway 60ml of 0.08M Mg²⁺ is what amount of Mg²⁺?

If it were 1 litre of 1M Mg²⁺, how much Mg²⁺ would you have?

If it were 60 ml of 1M Mg²⁺, how much Mg²⁺ would you have?

You've added some from the chloride, how much?

So you need to add how much from the phosphate (BTW, are you sure about that formula, the charges don't seem to balance to me, but that's possibly irrelevant)?

If you have a total of 1 mol needed, and you've already added 0.3 mol from some source, how much more do you need?

As you have a volume, and an amount for the phosphate you can then calculate a concentration.

If you have 34 mmol of, say KCl, in a total volume of 0.85 litres, what concentration is that?