#"%(w/w)"=("mass of solute")/("total mass of solution")xx100%#,
where the total mass of solution is the mass of the solute plus the mass of the solvent, in this case water.
#"10%(w/w) NaCl"=("mass of NaCl")/("100 g solution")xx100%#
Divide both sides by #"100%"#.
#"0.1 NaCl"=("mass of NaCl")/("100 g solution")#
Multiply both sides by #"100 g"#.
#"10 g NaCl"##=##"mass of NaCl"#
#"mass of water"="mass of solution"-"mass of NaCl"#
#"mass of water"="100 g"-"10 g"="90 g"#
Since the density of water is #"1 g/mL"#, #"90 g"# of water will have a volume of #"90 mL"#.
To prepare a #"10%(w/w) NaCl"# solution, mass out #"10 g"# #"NaCl"# and place it in a #"100-mL"# volumetric flask. Add about #"80 mL"# of water to the flask. Once the #"NaCl"# has dissolved, add more water up to the #"100-mL"# mark. If you don't have a volumetric flask, you can use a #"100-mL"# graduated cylinder, but it won't be as accurate.
