The percentage solution could be expressed in terms of weight percent (% w/w), volume percent (% v/v) and weight-to-volume percent (% w/v) units of solute present in 100 units of solution. For example a solution of 1.5% w/v NH4NO3, contains 1.5 gram of NH4NO3 in 100 mL of solution.
Percent by weight (% w/w)
In case of preparing a solution based on percentage by weight, one would simply determine what percentage was required (for example, a 20% by weight aqueous solution of sodium chloride) and the total quantity to be prepared. If the total quantity needed is 1 kg, then it would simply be a matter of calculating 20% of 1 kg which, of course is: 20 /100 * 1000 g/kg = 200 g NaCl/kg. Thus finally to bring the total quantity to 1 kg, it would be necessary to add 800g water.
Percent by volume (% w/v)
Preparation of solutions based on percent by volume it requires the calculation same as for percent by weight, except that calculations are based on volume. In simple terms one should plan that what percentage was desired (for example, a 20% by volume aqueous solution of sodium chloride) and the total quantity to be prepared in terms of volume. For example if 20 % is to be prepared for a total quantity of 1 liter, then it would simply be a matter of calculating 20% of NaCl in 1 liter, the formula can be written as:
20/100 * 1000 ml/l = 200 g NaCl/l.
Percent by volume (% v/v)
Volume percent or volume/volume percent most often is used when preparing solutions of liquids. This is typically only used for mixtures of liquids. Volume percent is relative to volume of solution, not volume of solvent. The advantage of volume/volume units is that gaseous concentrations reported in these units do not change as a gas is compressed or expanded. For example, 70% v/v rubbing alcohol may be prepared by taking 700 ml of isopropyl alcohol and adding sufficient water to obtain 1000 ml of solution (which will not be 300 ml).
Common Units for Reporting Concentration
|4.||Weight percent||grams solute 100 grams solution||% w/w
|5.||Volume percent||mL solute
100 mL solution
|6.||weight-to-volume percent||grams solute
100 mL solution
1. Percent by weight could be expressed as …………….
2. For preparation of 20 % NaCl by (w/v) ……………. grams of NaCl is to be dissolved in 1 L of water.
(a) 200 (b) 100 (c) 400 (d) 50
3. volume/volume percent most often is used when preparing solutions of …………….
4. Match the following:
(i) Molarity (a) N
(ii) Normality (b) m
(iii) Molality (c) M