- #1
bublik13
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Homework Statement
A buffer is made by dissolving 13.0g of sodium dihydrogen phosphate, NaH2PO4, and 15.0g of disodium hydrogen phosphate, Na2HPO4, in a litre of solution. What is the pH of the buffer?
a) 7.84
b)7.47
c)7.20
d)6.85
e)6.63
Homework Equations
pH = -log [H+]
The Attempt at a Solution
First, I'm not sure if this is right, but I assumed that NaH2PO4 dissassociates into Na+ and then H2PO4-, and then H+, H+, PO4 3- ions. Second I assumed that it dissassociates into Na+, Na+, H+ and PO4 3- ions.
2)
M(Na2HPO4) = 141.96
m=15.0g
n=.105664
3) M(NaH2PO4) = 119.98
m=13.0g
n=.10835 * 2 = 0.2167 (since hydrogen ion concentration doubles)
4) 0.105664+0.2167=0.322 mol H+
5) pH = -log(.322) = 0.492
This is completely off. Please help. I realize I have a completely wrong approach, but I don't know what else can be done for this problem.