Question

Question #72592

Answer 1

Here's what I got.

Explanation:

For starters, the thing to remember about neutral solutions is that they have a pH equal to `7` at room temperature. This means that you mislabeled the `"pH" = 7.00` solution as being basic, when in fact, it is neutral.

Now, a neutral solution kept at room temperature always has

`color(blue)(ul(color(black)("pH " + " pOH" = 14)))`

In terms of the concentration of hydronium cations, `"H"_3"O"^(+)`, which can also be represented as `"H"^(+)`, and hydroxide anions, `"OH"^(-)`, you have

`color(blue)(ul(color(black)(["H"_3"O"^(+)] * ["OH"^(-)] = 10^(-14))))`

A neutral solution must have

`["H"_3"O"^(+)] = ["OH"^(-)]`

which means that at `"pH" = 7.00`, you have

`["H"_3"O"^(+)] = ["OH"^(-)] = 1.0 * 10^(-7)"M"`

Consequently, an acidic solution will always have

`{("pH" < 7), ("pOH" > 7) :} " "` and `" " {(["H"_3"O"^(+)] > 1.0 * 10^(-7)"M"), (["OH"^(-)] < 1.0 * 10^(-7)"M") :}`

and a basic solution will always have

`{("pH" > 7), ("pOH" < 7) :} " "` and `" " {(["H"_3"O"^(+)] < 1.0 * 10^(-7)"M"), (["OH"^(-)] > 1.0 * 10^(-7)"M") :}`

Remember, an acidic solution has a higher concentration of hydronium cations than of hydroxide anions, and so its pH falls below `7.00`.

A basic solution has a higher concentration of hydroxide anions than of hydronium cations, and so its pH rises above `7.00`.

With this in mind, where would the solution that has

`["OH"^(-)] = 5.7 * 10^(-3)"M"`

be placed? Since it's clear that

`5.7 * 10^(-3) > 1.0 * 10^(-7)`

you should say that this solution is basic, not acidic. How about the solution that has

`["OH"^(-)] = 1.9 * 10^(-13)"M"`

where would that be placed? This time, you have

`1.9 * 10^(-13) < 1.0 * 10^(-7)`

so this solution is clearly acidic, i.e. its hydronium concentration is `> 1.0 * 10^(-7)"M"`.

Use this explanation to double-check the rest of your solutions.