Chemistry, help.
posted by Kati on .
Household bleach is 5.25% NaOCl by mass. Assume a density of 1.00g/ml. What is the pH of bleach? Hint: assume 100 ml. Answer: 10.7.
How does one approach this question?
Kati

NaOCl = 5.25% which means 5.25 g NaOCl + 94.75 g of water but since the density of the solution is 1.00 g/mL then that's 94.75 mL water.
moles NaOCl = 5.25 g NaOCl/molar mass NaOCl = 5.25/74.44 = 0.0705 moles NaOCl.
That is in 94.75 mL H2O or 0.09475 L so molarity is 0.0705/09475 = 0.744 M. check my arithmetic.
OCl^ + HOH ==> HOCl + OH^
Kb = Kw/Ka = (HOCl)(OH^)/(OCl^)
Look up Ka for HOCl. Kw = 1 x 10^14. In the equation you know (HOCl)=(OH^) so solve for (OH^), convert that to pOH, and convert that to pH.
Post your work if you get stuck. 
Ok, so I found the Ka and then found the Kb. I got 2.857x10^7. I then subbed that into the equation: [x^2]/[0.744x]...you can use the assumption rule to get rid of the minus x in the denominator. When I solved I got x=0.0000461043, and a pH of 3.34. What went wrong? The answer is supposed to be 10.7 which makes sense, since the products are basic.
Thanks
Kati 
I understand what I did! The 3.34 is the pOH thus, 14pOH is the pH which is 10.66 or 10.7 :)
Thanks very much!!
Kati 
Nothing went wrong except that you forgot to think about your answer. IF you look at the equation, what did you let X stand for? The answer is that you let it stand for (OH^). So you found the pOH (when you took the negative log) thinking that was the negative log of H^+. Just subtract that from 14 to get pH. 14 3.3 = 10.7. Voila! Ain't chemistry interesting? Besides that, it keeps one on one's toes.