SOLUTIONS, Expressions of Solution Concentrations,Sources of Error in the Preparation of Solutions

SOLUTIONS,Concentrations,Preparation

SOLUTIONS

Expressions of 

Solution Concentrations

Sources of Error in the 

Preparation of Solutions

 

SOLUTION

A solution is composed of a solvent which is the liquid, and a substance or the solute which is dissolved or diluted in the solvent. Molecules or ions of the solute are evenly distributed throughout the solvent.

Expressions of Solution Concentrations

There are several different ways in which concentrations of solutions can be expressed, and some of them are: 

1. Weight or mass per unit volume (w/v) This is used when a solid chemical is diluted in a liquid. It is usually expressed as grams per litre (g/L) or milligrams per millilitre (mg/ml). If concentration of a certain solution is given as 20 g/L, it means that there are 20 g of solute for every litre of solution

2. Volume per unit volume (v/v) This is used when a liquid chemical is diluted with another liquid. It is expressed as millilitres per millilitre (ml/ml) or millilitres per litre (ml/L). When 70 ml of alcohol is diluted to 100 ml with water, the concentration is expressed as 70 ml/100 ml or 0.7 ml/ml, or 70%. 

3. Weight per unit weight (w/w) This is another way of expressing concentration, though not commonly used. When a desired substance is a solid, and is mixed or diluted with another solid, the concentration is expressed as mass per unit mass, which means milligrams per milligram (mg/mg) or grams per gram (g/g), or grams per kilogram (g/kg).

4. Per cent solution A per cent solution usually means grams of solid per 100 ml of solution (g/100 ml ) or ml of liquid per 100ml (ml/100 ml; v/v). Per cent solution can be expressed as weight per unit volume per cent (w/v%) or volume per unit volume per cent (v/v %), depending on whether the solute is a solid or a liquid. For example, 70 per cent alcohol is 70 parts of alcohol and 30 parts of water (v/v %). 

5. Saturated solutions Saturated solutions are those solutions that contain maximum amount of solute that can normally be dissolved at a given temperature and pressure. The solubility of a solute, that is, the concentration of the solute in saturated solution at a given temperature, is constant. The solubility of a solid in a liquid usually increases as the temperature increases. 

6. Standard solutions Standard solutionsare those, the exact strength of which is known, and are used for comparing strengths of other similar solutions. Such solutions are used as reference solutions, and therefore, must be prepared with special care.

7. Mole per litre (Molar) solution This is a term that was known as molar solution. A mole per litre solution contains one mole of solute dissolved in and made up to 1 litre with solvent. One mole is the number of grams of a substance equal to its atomic or molecular weight. One millimole is the number of milligrams of a substance equal to its atomic or molecular weight. 

Preparation of mol/L solution A formula is very handy for easy and quick calculation: Required mol/L solution X molecular mass of the substance = number of grams to be dissolved in 1 litre of solution. 

e.g.To make 1 litre of solution (NaCl 1 mol/L)

Required mol/L concentration = 1 Molecular mass of NaCl = 58.44 Therefore 1 litre of NaCl, 1 mol/L contains 1 x 58.44 = 58.44 g of the salt dissolved in 1 litre of solvent. Usually the concentration of mol/L solution is written after the name of the substance, e.g., KH PO, 0.5M.

8. Normal solution This is a solution containing the equivalent mass of the solute in 1 litre of solution. The equivalent mass of a substance is the number of parts by weight which will combine with or displace 1 part by weight of hydrogen or 8 parts by weight of oxygen. 

Therefore, I normal solution

9. Isotonic solution Isotonic solutions are those that exert equal osmotic pressures, and in the medical sciences, that means equal osmotic pressure as blood serum. Isotonic or physiological saline is 8.5g in 1 litre of water.

Sources of Error in the Preparation of Solutions

Solutions accurately prepared, play a major role in laboratory investigation. The reliability of results in analytical methods depends to a large extent, on standard solutions.

Sources of error in the preparation of solution include: 

1. Impure chemicals It is a fact that nochemical is 100% pure, but much reliance is placed upon analytical grade chemicals that meet the required standard, specifications and are procured from a reputable manufacturer. Care is taken not to contaminate stocks of reagents. 

2. Accurate weighing Poor technique of weighing leads to serious errors in the preparation of solutions. Most errors of weighing emenate from poor arithmetic knowledge. 

3. Inaccurate apparatus "The use of faulty analytical balance or glassware contribute to errors in the preparation of solutions.

4. Improper techniques Most errors will be due to improper techniques. In the preparation of a solution, care should be taken to read calibration marks on volumetric apparatus and to ensure complete dissolution of solids and sufficient mixing. 

5. Deliquescent and hygroscopic substances Deliquescent substances, e.g., calcium chloride and potassium hydroxide, when exposed to air, absorb water and get dissolved in it. On the other hand, substances such as sodium carbonate and sulphuric acid, when exposed to air, absorb water but are not dissolved in it, are known as hygroscopic substances. These substances should be stored in air-tight containers with dessicants. They should be weighed quickly to avoid absorption of moisture from the air. 

6. Hydrated and anhydrous salts Salts which contain water of crystallisation are known as hydrated salts; and those which do not contain water of crystallisation are said to be anhydrous. The molecular weight of hydrated salts includes the weight of water of crystallisation and should be borne in mind while preparing solutions.