What is the pH of a solution containing 0.3 g/L OH-?

Question

What is the pH of a solution containing 0.3 g/L OH-?

Answer 1

You are given the starting units of grams per liter. We would like to have that in a useful concentration unit: mol/L for such a problem.

What do you know about pH of water? pH=14-pOH
We don't know pOH, but how can we find out? Well, pOH=-log[OH-] just like pH=-log[H+], the log being base 10.
Can you see what I'm going to do next?

We are given the concentration! And now you can see why we have to convert the units of it to mol/L, because that's what the brackets mean.
[OH-]=0.3g/L OH- * 1mol/(grams of O + grams of H), which cancels the g in the numerator with the g in the denominator and replaces the numerator with moles. The trick with these is to set up so that units cancel to leave your desired unit. Look up the gram values for oxygen and hydrogen on the periodic table. (Hint: the atomic weights.) Once you solve the concentration and plug it into the pOH formula listed above, you can use the fact the first formula I listed: pH=14-pOH

The answer should be to one significant figure. Good luck! Let me know if you have further trouble.

Answer 2

The molar mass of   OH^-   is  17 g.     So a concentration of 0.3 g/L  of OH^-  is  .3/ 17 = 0.0176 molar. 

 

The pOH is   then pOH  = log(.0176) = 1.75.     The corresponding pH is  14 - pOH =  12.25