[starswept]

Hello again,

Sorry for opening another topic, but I know some boards are strict about limiting each topic to 1 question, so I figured I'd be on the safe side.

Anyways, I'm working on the following question:

If 4.80 g of solid hydrogen chloride is dissolved in 250.0 mL of distilled water, then:
b) Determine the number of moles of hydrogen ions
c) Determine the concentration of hydrogen ions present in the solution
d) Determine the pH of the solution

For part b), I calculated 0.132 mol of H3O+, thus 0.132 mol of H+.

However, for part C, I'm uncertain how to calculate the concentration. I know I should probably use the formula Molar concentration (mol/L) = Amount of solute (mol)/Volume of solution (L)
However, I don't have the volume of the solution, only the volume of the solvent, and it is tricky to convert g into mL. Thus, I was considering using this formula:

m/v (g/mL) = Mass of solute (g)/Volume of solution (in mL) x 100

But that doesn't make sense since I need the concentration in mol/L in order to calculate the pH in part D of the question. Thus, I have managed to thoroughly confuse myself. Any advice would be greatly appreciated! Thank you.

[enahs]

This is where common laboratory practice and assumptions come in. First, you do not add 4.8g of HCl to 250.0mL of water. You add 4.8g of HCl, and you then you add water to bring the total solution volume up to 250.0mL. By doing it this way, you are actually adding less then 250.0mL of water.

So your final volume is just your solvent volume. Now, this is an assumption based on other things, but the above is just one way to rationalize it.

[ARGOS++]

Dear Starswept;

After consulting my table I can tell you that in either case you are doing a “mistake” of less than 1%, because the density of the solution is very close to 1.008g/ml.
If you calculate it back to the volume change it means also less than 1%.
Conclusion:
It’s quite legal to use density 1.0g/ml for your calculations, and that means that it is quite independent if you calculate in ml or in gram.
Now the calculation becomes nearly simple, especially as you already have one correct result.

It seems that you have not forgotten what the meaning/consequence of a strong Acid is.


Good Luck!
                   ARGOS⁺⁺

[Borek]

Sorry for opening another topic, but I know some boards are strict about limiting each topic to 1 question, so I figured I'd be on the safe side.

To be on the safe side - just read forum rules. So far you are OK

[ARGOS++]

Dear Enahs;

I’m sure absolutely every good Chemist (- and me, too -) prefer your preparation method for solids in solution not only tremendous, - But for HCl?, - or “solid” HCl?  (Mp. -114.2°C)

I think in this case the laboratory practise will go an alternative way:
Add the HCl as Gas to ~200ml of dest. water and add finally dest. water till to the mark (250ml).
(And even that can be quite tricky in case of HCl, if you are not “trained”!!)

Good Luck!
                   ARGOS⁺⁺