Answer:
Molarity tells you how many moles of solute, which in your case is potassium iodide,
KI
, are dissolved in exactly one liter of solution.
The first thing to do here is use the molar mass of potassium iodide to figure out how many moles you have in that sample
5.85
g
⋅
1 mole KI
166.0
g
=
0.03524 moles KI
Now, you know that this many moles of potassium iodide are dissolved in
0.125 L
of solution. Since you're interested in figuring out how many moles would be present in
1 L
of solution, all you have to do is scale up your current volume by a factor of
1
L
0.125
L
=
8
The volume goes up by a factor of
8
, which can only mean that the number of moles of solute must go up by the same factor
1
L solution
⋅
0.03524 moles KI
0.125
L solution
=
0.282 moles KI
Since you have
0.282
moles of potassium iodide in
1 L
of solution, it follows that the molarity will be
molarity
=
∣
∣
∣
∣
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯
a
a
0.282 mol L
−
1
=
0.282 M
a
a
∣
∣
−−−−−−−−−−−−−−−−−−−−−−−−−−
The answer is rounded to three sig figs.