Question

3)Which substance has aqueous solution with ph *less than* 7? a) Hydrochloric acid (b) sodium hydroxide (c) hydrosulfuric acid , (d) Acetic acid

Answer 1

All the substances EXCEPT `(b)` have `pH<7`.........

Explanation:

By definition, `pH=-log_10[H_3O^+]`...and thus the LOWER (or more negative) the `pH` the GREATER the concentration of `H_3O^+`.

In aqueous solution, in which we ASSUME all these solvents are dissolved, the following equilibrium operates:

`2H_2Orightleftharpoons H_3O^+ + HO^-`

And, by careful measurement, the following equilibrium is established and quantified....

`K_w=10^-14=[H_3O^+][HO^-]`...under standard conditions...

And we take `log_10` of BOTH sides...

`underbrace(log_(10)10^-14)_(-14)=log_10([H_3O^+][HO^-])=log_10[H_3O^+]+log_10[HO^-]`

And so `-14=underbrace(log_10[H_3O^+])_(-pH)+underbrace(log_10[HO^-])_(-pOH)` ... or...

`14=pH+pOH`...

And thus THE GREATER `[H_3O^+]` the LOWER the `pH`. Acetic acid and `H_2S` are weak acids, however, the extent of equilibrium reduces the `pH<7`...

What are `pH` for `1.0*mol*L^-1` `HCl`, and for `10*mol*L^-1` `HCl`?

On the other hand, were we to have `1.0*mol*L^-1` `NaOH`..`[HO^-]=1.0*mol*L^-1`...and `pOH=-log_(10)1=0`...i.e. `pH=14`. Capisce?