Measuring Substances

To measure chemicals requires different strategies depending on the state of matter.

- Solids: Weigh out the substance
- Liquids: Measure volume
- Solutions: Use the solution concentration to calculate moles

# Running Example

### Example:

1L Aqueous Solution has 571.6 gm of H_{2}SO_{4} and has a density of 1.329 g cm^{-3}. Determine the molarity, molality, mole fraction and mass percentage.

Density = Mass/Volume. Therefore Mass = Density * 1L = Density.

Therefore Mass = 1.329 Kg.

# Measuring Concentration

Concentration of X is written as [X] -> e.g. NH_{3} [2]

There are many different ways of expressing concentration

## Molarity/Molar Concentration

- Molarity = Moles per Litre
- Concentration = Amount of Solute/Volume of Solution (C = n/v)
- However this changes with temperature, as Volume increases/decreases

### Example

- H
_{2}SO_{4}has an atomic weight of 98.08 (g). - 571.6/98.08 = 5.827
- 5.827/Litres of Solution * 1L = 5.827 Mol L
^{-1}

## Molality

- Molality = Moles per kilogram of solvent
- Concentration = Amount of Solute/
**Mass of Solvent**

### Example

- 1.329 Kg - 571.6 g = 757.4 gm of H
_{2}O - 5.827/0.7574 = 7.69 Mol Kg
^{-1}

## Molar Fraction

- Molar Fraction = Not a unit, just what fraction of moles is X
- Concentration = Amount of Solute/Total Moles

### Example

- Weight of 757.4/18.016 (mass of H
_{2}O) = 42.04 moles of water - Mole Fraction = 5.827/(5.827+42.04) = 0.12

## Percentage Composition by Mass

- Percentage Composition = What percentage of the mass is X
- Concentration = (Mass of Solute/Total Mass) * 100

### Example

- 571.6 gm/1329 gm = 0.43
- 0.43 * 100 = 43%

page revision: 5, last edited: 14 Mar 2013 06:37