Calculate pH of a Weak Acid

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Here I show how to calculate the pH of a solution made with a weak acid

Summary: Use Ka to determine how much of your original acid dissociates to give H+. Then you can plug that concentration of H+, [H+], right into the pH formula.

Category:

Education

Tags:

• chemistry
• high school
• acids
• bases
• pH
• strong acid
• logarithm
• acidity
• chemistNATEchem

[sizzlewithabir]

Doesn't ionisation of water also give H+? Why don't we have to add that with the conc. of the H+ obtained by the dissociation of the original acid?

[chemistNATE]

@sizzlewithabir Only account for that if you have:

1. A very low concentration of acid

2. A very weak base

The ionization of water only plays a role when the concentration of H+ from the acid is close to 1.0x10^-6 mol/L, since the ionization of water contributes 1.0x10^-7 mol/L...

[Jfrancis221]

My book says that HF has an acid ionization of 6.8X10^-4 ...

[chemistNATE]

@Jfrancis221 Good eye. Acids will have different Ka values at different temperatures ... so not all tables will have the same value for Ka. Just use the one in your book, and follow the procedure in the video :)

I can find some sources that agree with you, and some that say it's closer to the value that I used. Again, just use the Ka your teacher gave you.

Thanks for watching!

[sizzlewithabir]

@chemistNATE The concentration of an ion from a weak acid/base has practically no effect on the conc.of the same ion from an acid/base much stronger than it. I get it now. Thanks.

[maniraneshgh]

nice

[TheIkitta]

Oh never mind, I just found the other video with the result, thank you! very helpful :)

[TheIkitta]

It would be super useful if you could give us answer for pH at the end. I'm doing it along with you and wanted to know it you got pH=2.1

[robomanus]

THANKS for the video!

[Dragogoel]

@lifeDotGov, ofcourse it was invented at your home , since you live in India.