20 mL of 0.1 M HCL is added to the 30 mL of 0.1 M NaOH. To this solution extra 50 mL of water was added. What is the molarity of the final solution formed?

Moles of HCl in 20 mL of 0.1 M is:

$\mathrm{moles}=0.1 \mathrm{M}\times 0.02 \mathrm{L}=0.002 \mathrm{moles}$

Moles of NaOH in 30 mL of 0.1 M is:

$\mathrm{moles}=0.1 \mathrm{M}\times 0.03 \mathrm{L}=0.003 \mathrm{moles}$

Reaction is:

$\mathrm{HCl}+\mathrm{NaOH}\to \mathrm{NaCl}+{\mathrm{H}}_2\mathrm{O}$

Molarity is calculated as:

$\frac{\mathrm{number} \mathrm{of} \mathrm{moles} \left(\mathrm{left}\right)}{\mathrm{volume} \mathrm{in} \mathrm{L}}=\frac{0.001}{100 \mathrm{mL}\times {\displaystyle \frac{1 \mathrm{L}}{1000 \mathrm{mL}}}}=0.01 \mathrm{M}$