Question #72592

1 Answer
Nov 12, 2016

Answer:

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.