[hamil]

I am trying to make my own alkaline water using baking soda and distilled water.
First I measure out 0.018 moles of baking soda as follows
(0.25 tsp)(1ml/.202 tst)(1.2 grams NaHCO3/ml)(1 mole NHCO3/1 gram NaHCO3) = .018 moles NaHCo3
I measure out 0.236 liters of distilled water i.e. 1 cup.
The molarity of the NaHCO3 is not M= 0.018 moles NaHCO3/0.236 liters of water = 0.076
The pOH of the solution is –log(0.076) = 1.12 thefore the pH is 14 – 1.12 = 12.88.
Note: the molarity of [OH] should be the same as the molarity of NaHCO3.
However, when I put ¼ tsp baking soda into as cup of distilled water the pH measures only 8.0.
What is wrong here? Is the baking soda not disassociating?

NaHCO3 + H2O -> Na +  OH + CO2
Do I have to heat the mixture to make the NaHCO3 dissassociate?

[AWK]

pH of NaHCO₃ solutions is always close to 8. Your calculations are wrong.
Solutions of Na₂CO₃ show pH close to 13.

[hamil]

It seems that baking soda, NaHCO3 does not dissociate very well hence low pH. However, I read that I can convert baking soda to washing soda, Na2CO3 by heating it and this will dissociate and my pH might be correct. Yes?

[Dan]

Note: the molarity of [OH] should be the same as the molarity of NaHCO3.

Why do you assume this?

[AWK]

NaHCO₃ start decomposition  ~80 C, hence boiling solution an hour or more should convert bicarbonate into carbonate.

[hamil]

I thought about biling the solution but I found on the Internet that you can bake the NaHCO3 to get Na2CO3. However, I am not sure this will give me the pH I am wanting since there is no OH in Na2Co3.

[AWK]

pH of Na₂CO₃ solutions is concentration dependent, but eg. 0.5 M solution show pH ~12. for pH greater ~12.5 you need strong base, eg NaOH.
Have you heard about salt hydrolysis?

[hamil]

Actually, this started when I wanted to make my own alkaline water with a PH of about 9.5, not 12.5. When I set up a sample problem, my (erroneous) calculations showed I should get a pH of around 12. I was surprised to get only about 8. Now I know the reason. There are some recipes on the Internet about how to make your own alkaline water using baking soda.  I will have to check them out to see if they really work.

[hypervalent_iodine]

I thought about biling the solution but I found on the Internet that you can bake the NaHCO3 to get Na2CO3. However, I am not sure this will give me the pH I am wanting since there is no OH in Na2Co3.

NaHCO3 doesn't have OH- either. It doesn't matter, as it generates OH- when in aqueous solution.

The concentrations you would need would be relatively high if your purpose here is to consume the water (I assume that's what this is for?). Putting aside the fact that you aren't getting any health benefits from drinking it, have you ever tasted baking soda? It's not pleasant. I can't imagine that a solution of the stuff would be any better.