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]/[0.744-x]...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, 14-pOH 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.
To approach this question, you need to understand that the pH of a solution can be determined based on the concentration of hydrogen ions (H+). Bleach, which contains sodium hypochlorite (NaOCl), will dissociate into sodium ions (Na+) and hypochlorite ions (OCl-).
To find the pH of bleach, you can follow these steps:
1. Convert the mass percentage of NaOCl to grams:
- Assume you have 100 mL of bleach, which has a density of 1.00 g/mL, so the mass of the bleach is 100 grams.
- The bleach is 5.25% NaOCl by mass, so the mass of NaOCl in the bleach is 5.25 grams (0.0525 x 100 grams).
2. Convert the mass of NaOCl to moles:
- Use the molar mass of NaOCl, which is approximately 74.44 g/mol.
- Divide the mass of NaOCl by its molar mass: 5.25 grams / 74.44 g/mol = 0.0705 moles.
3. Calculate the concentration of hypochlorite ions (OCl-):
- Since NaOCl dissociates into one Na+ ion and one OCl- ion, the concentration of OCl- will be the same as the concentration of NaOCl.
- Divide the moles of NaOCl by the volume in liters: 0.0705 moles / 0.1 L = 0.705 M (M = moles per liter).
4. Calculate the pOH of the solution:
- Since the bleach is basic, we will first calculate the pOH, which can be used to find the pH.
- Use the formula: pOH = -log10[OH-]
- For a concentration of 0.705 M OCl-, we can assume the concentration of OH- is the same.
- Take the negative logarithm (base 10) of the OH- concentration: -log(0.705) ≈ 0.150.
5. Calculate the pH of the solution:
- The pH and pOH are related by the equation: pH + pOH = 14.
- Subtract the pOH from 14 to find the pH: 14 - 0.150 ≈ 13.850.
Therefore, the pH of the bleach is approximately 13.850. This value might differ slightly based on the actual density and concentration of NaOCl in the bleach.