# Problem: A 0.5890 g sample of impure magnesium hydroxide is dissolved in 103.0 mL of 0.2040 M HCl solution. The excess acid then needs 19.88 mL of 0.1036 M NaOH for neutralization.Calculate the percent by mass of magnesium hydroxide in the sample, assuming that it is the only substance reacting with the HCl solution.  Express your answer using four significant figures.

###### FREE Expert Solution

Recall that at the equivalence point of a titration:

Also, recall that moles = molarity × volume

There are 2 equivalence points:

1. Mg(OH)2  + 2HCl  → MgCl2 + 2H2O
2. HCl + NaOH → NaCl + H2O

moles base = moles acid - moles HClexcess (2nd equiv.)

$\overline{){{\mathbf{moles}}}_{{\mathbf{acid}}}{\mathbf{=}}{\left(\mathbf{MV}\right)}_{{\mathbf{acid}}}}$

For 1st equiv. point:

• 2 moles HCl react with 1 mole Mg(OH)2
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###### Problem Details

A 0.5890 g sample of impure magnesium hydroxide is dissolved in 103.0 mL of 0.2040 M HCl solution. The excess acid then needs 19.88 mL of 0.1036 M NaOH for neutralization.

Calculate the percent by mass of magnesium hydroxide in the sample, assuming that it is the only substance reacting with the HCl solution.  Express your answer using four significant figures.