How do you figure out the final molar concentration when two substances are mixed

Here is a question to use an example

50.0mL of 0.20M NaOH and 50.0mL of 0.20M NaOH

For this particular question, you are adding together two volumes of the same compound that have the same concentration. So the concentration is the same. Final concentration = 0.20 M


Say you were combining
50.0mL of 1.0 M NaOH and 100.0mL of 0.20M NaOH

first work out how many moles of NaOH are in the stated volume of each solution
moles = molarity x Litres

moles NaOH in 50.0 ml of 1.0 M = 1.0 M x 0.0500 L = 0.050 moles
moles NaOH in 100 ml of 0.20 M = 0.20M x 0.100 L = 0.020 moles

Total moles NaOH = 0.050 mol + 0.020 mol = 0.070 moles
Total volume = 50 ml + 100 ml = 150 ml = 0.150 L

Final concentration = moles / litres
= 0.070 mol / 0.150 L
= 0.47 M NaOH