When 8.0 grams of sodium hydroxide is dissolved in sufficient water to make 400. mL of solution, what is the concentration of th
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0.040 mol / dm³. (2 sig. fig.)
<h3>Explanation</h3> in this question acts as a weak base. As seen in the equation in the question,
produces
rather than
when it dissolves in water. The concentration of
will likely be more useful than that of
for the calculations here.
Finding the value of from pH:
Assume that ,
.
.
Solve for :
Note that water isn't part of this expression.
The value of Kb is quite small. The change in is nearly negligible once it dissolves. In other words,
.
Also, for each mole of produced, one mole of
was also produced. The solution started with a small amount of either species. As a result,
.
,
,
.