We use the relationship:

#"Molarity"="Moles of solute"/"Volume of solution"#, and thus to get the number of #"moles"# we multiply the #"volume"# by the #"molarity"# to get an answer in #"moles".#

And for your problem, there are:

#5xx10^2*mLxx10^-3*mL*L^-1xx2.0*mol*L^-1=1*mol# with respect to #KNO_3#.

Note the use of the units in the calculation. We wanted an answer in #mol#; the calculation gave us an answer in #"mol"#. Chemists (and physical scientists) often include units in their calculations to ensure that they perform the right order of operations: #"do I divide or do I multiply?"# Anybody can make a mistake, and everybody has.

And since #"moles"="mass of solute"/"molar mass of solute"#, there are #1*cancel(mol)xx101.10*g*cancel(mol^-1)=101.1*g#