Solutions

Definitions A solution is a homogeneous mixture Solvent is the liquid in which the solute is dissolved an aqueous solution has water as solvent A saturated solution is one where the concentration is

at a maximum - no more solute is able to dissolve. A solution is composed of a solvent which is the

dissolving medium and a solute which is the substance dissolved.

In a solution there is an even distribution of the molecules or ions of the solute throughout the solvent.

The concentration of a solution can be expressed in a variety of ways (qualitatively and quantitatively).

Formerly, the concentration of a solution can be expressed in four ways:– Molarity(M): moles solute / Liter solution – Mass percent: (mass solute / mass of

solution) * 100

– Molality (m) - moles solute / Kg solvent

– Mole Fraction(cA) - moles solute / total moles

solution

Qualitative Expressions of Concentration

A solution can be qualitatively described as dilute: a solution that contains a small

proportion of solute relative to solvent, or concentrated: a solution that contains a

large proportion of solute relative to solvent.

Semi-Quantitative Expressions of Concentration A solution can be semi-quantitatively described as Unsaturated: a solution in which more solute will

dissolve, or Saturated: a solution in which no more solute will

dissolve. The solubility of a solute is the amount of solute

that will dissolve in a given amount of solvent to produce a saturated solution.

Percent Composition (by mass)

We can consider percent by mass (or weight percent, as it is sometimes called)

We need two pieces of information to calculate the percent by mass of a solute in a solution:

The mass of the solute in the solution. The mass of the solution. Use the following equation to calculate percent by

mass:

Molality

Molality, m, tells us the number of moles of solute dissolved in exactly one kilogram of solvent. (Note that molality is spelled with two "l"'s and represented by a lower case m.)

We need two pieces of information to calculate the molality of a solute in a solution:

The moles of solute present in the solution. The mass of solvent (in kilograms) in the

solution. To calculate molality we use the equation:

Molarity Molarity tells us the number of moles of solute in

exactly one liter of a solution. (Note that molarity is spelled with an "r" and is represented by a capital M.)

We need two pieces of information to calculate the molarity of a solute in a solution:

The moles of solute present in the solution. The volume of solution (in liters) containing the

solute. To calculate molarity we use the equation:

Mole per liter solutions

A mole per litre (mol/l) solution contains one mole of a solute dissolved in, and made up to 1 litre with solvent.

Mole:

A counting unit Similar to a dozen, except instead of 12, it’s 602

billion trillion 602,000,000,000,000,000,000,000 6.02 X 1023 (in scientific notation) A Mole of Particles Contains 6.02 x 1023 particles 1 mole C= 6.02 x 1023 C atoms 1 mole H2O = 6.02 x 1023 H2O molecules 1 mole NaCl = 6.02 x 1023 NaCl “molecules” The mole is the SI base unit that measures an

amount of substance. One mole contains Avogadro's number

(approximately 6.023×1023) (number of atoms or molecules).

Preparation of mol/l solution

To preparation a mol/l solution, use the following formula:

Number of grams to be dissolved in 1 litre of solution= Required mol/l solution X Molecular mass of substance

Example

Make a solution of 50 ml of sodium chloride, 0.15 mol/l:

Required mol/l concentration= 0.15 Molecular mass of NaCl= 58.44 Therefore 50 ml NaCl, 0.15 mol/l contains:

0.15X 58.44 X 50 = 0.438 g of the chemical . 1000

substance dissolved in 50 ml of solvent.

Make liter of sodium chloride (NaCl), 1 mol/l

To make 1 litre of sodium chloride (NaCl), 1 mol/l:

Required mol/l concentration= 1 Molecular mass of NaCl= 58.44 Therefore 1 lit NaCl, 1 mol/l contains: 1 X 58.44= 58.44 g of the chemical dissolved

in 1 litre of solvent.

To make 1 litre of sodium chloride, 0.15 mol/l (physiological saline)

Conversion a percentage solution into a mol/l solution:

By the following formula: Mol/l solution = g% (w/v) solution X 10 Molecular mass of the substance Examples:To convert a 4% w/v NaOH solution into a mol/l solution:Gram % solution = 4Molecular mass of NaOH= 40Conversion to mol/l= 4X10 = 1 40 Therefore 4% w/v NaOH is equivalent to NaOH, 1 mol/l solution.

Convert a 0.9% w/v NaCl solution into a mol/l solution

Conversion of a normal solution into a mol/l solution:

By the following formula:

mol/l solution= Normality of solution

Valence of substance Examples:

Convert 0.1 N (N/10) HCl into a mol/l solution:

Normality of solution= 0.1

Valence of HCl= 1

Conversion to mol/l= 0.1/1= 0.1 Therefore 0.1 N HCl is equivalent to HCl, 0.1mol/l

solution.

To convert 1 N Na2CO3 into a mol/l solution (valence =2)

How to dilute solutions and body fluids

In the laboratory it is frequently necessary to dilute solutions and body fluids to reduce its concentrations.

Diluting solutions:A weaker solution can be made from a stronger solution by using the following formula:Volume (ml) of stronger solution required= R X V OWhere: R= concentration of solution required V= volume of solution required. O= strength of original solution.

Examples: To make 500 ml of NaOH, 0.25 mol/l

from a 0.4 mol/l solution:

C= 0.25 mol/l, V= 500 ml, S=0.4 mol/l ml of stronger solution required:

0.25 x 500 = 312.5 ml

0.4 Therefore, measure 312.5 ml NaOH,

0.4 mol/l and make up to 500 ml with distilled water.

Example 1

Make 100 ml glucose, 3 mmol/l in 1 g/l benzoic acid from glucose 100 mmol/l solution :

C= 3 mmol/l V= 100 ml S= 100 mmol/l ml of stronger solution required= 3 x 100 = 3

100 Therefore, measure 3 ml of glucose, 100

mmol/l and make up to 100 ml with 1 g/l benzoic acid.

Example 2 To make 500 ml H2SO4, 0.33 mol/l from concentrated

H2SO4 which has an approximate concentration of 18 mol/l:

C= 0.33 mol/l, V= 500 ml S= 18 mol/l ml of stronger solution required= 0.33 x 500 = 9.2

18 Therefore, measure 9.2 ml conc. H2SO4, and slowly add

it to about 250 ml of distilled water in a volumetric flask. Make up to 500 ml with DW.

Make 1 litre of HCl, 0.01 mol/l from a 1.0 mol/l solution:

Diluting body fluids and calculating dilutions

To prepare a dilution or series of dilutions of a body fluid:

Examples: To make 8 ml of 1 in 20 dilution of blood:

Volume of blood required= 8/20 = 0.4 ml Therefore, to prepare 8 ml of a 1in 20 dilution, add 0.4 ml of blood to 7.6 ml of diluting fluid.

2- To make 4 ml of a 1 in 2 dilution of serum in physiological saline:

Volume of serum required= 4/2 = 2 ml

Therefore, to prepare 4 ml of a 1in 2 dilution, add 2 ml

of serum to 2 ml of physiological saline.

To calculate the dilution of a body fluid

Examples:

1- Calculate the dilution of blood when using 50 µl of blood and 950 µl of diluting fluid:Total volume of body fluid and diluting fluid= 50+950=1000 µlTherefore, dilution of blood: 1000/50= 20i. e. 1 in 20 dilution.

2- Calculate the dilution of urine using 0.5 ml of urine and 8.5 ml of diluting fluid (physiological saline):Total volume of urine and diluting fluid= 8.5+0.5=9 mlTherefore, dilution of urine: 9.0/0.5= 18i. e. 1 in 18 dilution.