# Destroyer Gen CHEM QUESTION

#### csulapredental

Guys i have a question about calculating ph problem.. I've been doing this one weird method for every one of those problems and it works.. i want to run it by u guys and see if this is even a thing or I'm just getting lucky lmao

so the question states: What is the pH of 1.0 x 10^-4 M Ca(OH)2 solution
1. this guy has an OH in it so [OH] = 1.0 x 10^-4
2. pOH is going to be 3 ish or 4 ish ...
3. pH is going to be 14-3 = 11 or 14-4 = 10
4. some number between 10 and 11... and the answer is 10.3

is this normal? is this actually a thing? am i just getting lucky.. PLEASE because if it works for all like i wanna keep using it.

#### Charles_Darwin

2+ Year Member
Guys i have a question about calculating ph problem.. I've been doing this one weird method for every one of those problems and it works.. i want to run it by u guys and see if this is even a thing or I'm just getting lucky lmao

so the question states: What is the pH of 1.0 x 10^-4 M Ca(OH)2 solution
1. this guy has an OH in it so [OH] = 1.0 x 10^-4
2. pOH is going to be 3 ish or 4 ish ...
3. pH is going to be 14-3 = 11 or 14-4 = 10
4. some number between 10 and 11... and the answer is 10.3

is this normal? is this actually a thing? am i just getting lucky.. PLEASE because if it works for all like i wanna keep using it.
Could you post the question number? I can help you out but in general although that way works sometimes it's not always going to work

#### DSNCSUN

2+ Year Member
It has two mols of OH though

#### Charles_Darwin

2+ Year Member
^ that's where it wouldn't work - if they gave you 10.3 or 10.6 you wouldn't know the correct answer. Chad's videos explains it pretty well though actually

OP
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#### csulapredental

i
^ that's where it wouldn't work - if they gave you 10.3 or 10.6 you wouldn't know the correct answer. Chad's videos explains it pretty well though actually

its question 76 but its from a 2013 book idk if they are the same,

OP
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#### csulapredental

^ that's where it wouldn't work - if they gave you 10.3 or 10.6 you wouldn't know the correct answer. Chad's videos explains it pretty well though actually
they never give anything that close to each other tho i haven't seen any that close??

#### Charles_Darwin

2+ Year Member
they never give anything that close to each other tho i haven't seen any that close??
Your method will work for only the simplest of pH questions - if they had said 4 x 10 ^4, would you have been able to solve it? I don't mean to cause anxiety, just saying that with your method there is a risk that it might not work - I wouldn't feel comfortable using it. But if you check out chad's explanation for it (i'm sure it's on SDN too) it's actually very similar to your method and won't take you that long to learn! I'd explain it now but it takes a while and I have my own DAT very soon haha

#### Mountain_man7

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since there is 2(OH) you mutiple it by 2 which give you 2 x 10^-4

if you watched chad he show you how to solve it.

1 x 10^-3 = 3
3.15 x 10^-4 = 3.5
1 x 10^-4 = 4

the model above is used to calculate ph according to chad. just use that model and you can solve all the ph problems.

since 2 x 10^-4 fall between 1 x10^-4 and 3.15 x 10^-4, the poh must be between 3.5 and 4. so the ph is 14- 3.7 (any number between 3.5 and 4) = 10.3

#### Mountain_man7

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also i would recommend you do fast math on paper. on my DAT they didn't give me a calculator. it took me almost 5 minutes to solve the percentage of elements problem. that was pretty bs.