[pleasehelp123]

Hard water has high concentrations of ca2+ and mg2+.  the total water hardness, reported in ppm CaCO3, can be determined via titration with EDTA and roughly categorized using the following scale:


Soft: 0-60 mg/L
Moderately hard: 61-120 mg/L
Hard: 121-180 mg/L
Very hard: >181 mg/L

a) if a 50 mL sample of hard water (150mg/L) is buffered to a pH of 10.00 and titrated using 0.005M EDTA, what will be pCa2+ after 17.5 mL of EDTA is added?

I am really having trouble with this.  i know that ppm = mg/L. but i am having trouble finding the right equation to start with.  
is it CaCO3 + H2O + EDTA -->  CoEDTA3 + Ca2+ ??
i calculated that there are 8.75*10^-5 mol EDTA.  and do I need to find the concentration of -OH? or of H+ with the pH given to me?
PLEASE HELP. i've been working on it for an hour

b) What's the hardness level of a 50.00mL water sample, buffered to a pH of 8, that requires 17.28mL 0.0025M EDTA to reach the endpoint?


ANY HELP WOULD BE APPRECIATED PLEASE!

[Borek]

Water doesn't contain CaCO₃, it contains Ca²⁺, which reacts with EDTA^4- on 1:1 basis:

Ca²⁺ + EDTA^4- <-> CaEDTA^2-

You report results AS IF solution contained CaCO₃ - for each mole of Ca²⁺ present there would be exactly one mole of CaCO₃.