Solutions

In Chemistry, a solution is a homogeneous mixture which consists of a solute, the substance that is dissolved, and a solvent, the substance in which the solute is dissolved.

Nature of Solutions

When a substance dissolves in another substance, it is said to be soluble. For example, when you dissolve sugar in water, therefore we can say that sugar is soluble in water. On the other hand, a substance that cannot be dissolved in another substance is said to be insoluble. For example, sand and rocks are insoluble in water.

Liquids which are completely soluble with each other are said to miscible, liquids which that don't mix with each other are said to be immiscible. A very good example of this is oil and water.

Types of Solutions According to:

A. Phase

1. Solid Solutions
2. Liquid Solutions
3. Gaseous Solutions

B. Saturation

1. Saturated Solutions- a solution wherein the maximum amount of solute is dissolved in the given solvent at a stated temperature.
2. Unsaturated Solutions- this is a type of solution wherein the solvent can still dissolve additional solute particles.
3. Supersaturated Solutions- a solution that contains more solute particles than the solvent can normally held at a stated temperature.

C. Concentration

1. Diluted- in this type of solution, only little amount of solute is dissolved in a solution.
2. Concentrated- unlike in diluted solutions, large amount of solute is dissolved in a concentrated solution.

Expressing Concentration of Solutions Quantitatively

• Percentage Composition

One of the simplest ways to express concentrations of solutions is through mass or volume percent. It is the percentage of volume or mass of a solute dissolved in a definite mass or volume of solution.

• Mole Fraction

Mole fraction is the ratio of the number of moles of one component of a solution and the total number of moles of all the components.

• Parts per Million (ppm)

In expressing very dilute concentrations we use the unit parts per million.

• Molarity (M)

The ratio of the number of moles of solute to the volume of the solution in liters is called molarity.

• Molality (m)

Molality is the alternative unit of concentration. It is the ratio of the number of moles of the solute divided by the mass of solvent in kilograms.

Concentration Units for Solutions

Units	Symbol	Definition
Mass percent	% m/m,	(masssolute/ masssolution)x100%
Volume percent	% V/V	(volume solute/ volumesolution)x100%
Mole fraction	x	masssolute/ (masssolute + masssolvent)
Parts per million	ppm	Volumecomponent/ Volumesolution)x106
Molarity	M	molesolute/ volumesolution
Molality	m	molesolute/ masssolvent

Sample Problems:

• What masses of sodium hydroxide (NaOH) and water are needed to make 190.0g of 34% 0f solution?

                      Solution:

                              Let x be the mass of KCl and

                                     190.0g - x be the mass of H₂O

                                     x/190.0g = 34%

                                                    x = 64.6g

                               190.0g-64.6g = mass of H₂O

                                       mass of H₂O = 125.4g

                                         mass of KCl = 64.6g

• A 25.0% phosphoric acid (H₃PO₄) solution has a density of 0.75g/mL. How many grams of H₃PO₄ are contain in 2.5L of this solution?

                     Given:

                              d= 0.75g/mL

                              V= 2.5 L or 2,500mL

                              mass%= 25%

                     Solution:

                              d= m/V

                                    *by manipulating this equation, we'll have;

                              m= dV

                              m= (0.75g/mL)(2,500mL)

                              m= 1,875g

                                    *Using the formula for mass percent: (mass_solute/ mass_solution)x100%, 

                                             calculate for the mass of H₃PO₄.

                              

                                     *Let x be the mass of H₃PO₄.

                              x/1,875g= 25%

                                            x= (1,875g)(0.25)

                                            x= 468.75g

• Calculate the molality of a 7.95 M ethanol (C₂H₅CH), whose density is 0.927g/mL provided that the volume of the solution is 1.5L and the molar mass (MM)of C₂H₅OH is 46.07g/mol.

                     Solution: 

                                                  d= mass_solution/V

                              mass_solution= dV

                              mass_solution= (0.927g/mL)(1,500mL)

                              mass_solution= 1,390.5g~mass of the solution

                              

                                       *For the mass of the solute, use the formula for Molarity:

                              M= mol_solute/V_solution

                                      *Substitute the formula for moles (n) which is n= mass_solute/MM_solute

                              M= (mass_solute/MM_solute)/V_solution

                                       *Substitute the given values.

                                                          sustituting...

                                     7.95M= (mass_solute/46.07g/mol.)/1.5L

                              mass_solute= (7.95M)(46.07g/mol.)(1.5L)

                              mass_solute= 549.38g

                                         *To find the mass of the solvent, 

                                                   subtract the mass of the solute from the mass of the solution:

                              mass_solvent= 1,309.5g-549.38g

                              mass_solvent= 760.12g or 0.76012kg

                                          *Finally, find the molality using:

                              m= mole_solute/ mass_solvent

                              m= (549.38g/46.07g/mol.)/ 0.76012kg

                              m= 15.69mol/kg

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Dilution

Is the process of reducing the concentration of a solute in solution, usually simply by mixing with more solvent.

 Stock solutions in the laboratory are of known molarity. In your experiments, you may need to prepare dilute solutions from concentrated solutions. Bear in mind that the number of moles of solute does not change when a solution is diluted.

no. of mol before dilution = no. of mol after dilution

          From the definition of molarity,

                    M = mol of solute (n) ; n(solute) = M x V

                              L of sol’n (V)

          Since the total number of moles of solute does not change,

M1V1 = M2V2

Solubility

The solubility of a substance is the amount of substance that dissolves in a given amount of solvent at a given temperature to produce a saturated solution. Solubility is usually expressed in grams of solute in 100g of solvent.

When you dissolve ordinary table salt in about 100g of water, you know from experience that it readily dissolves to give a clear solution. However, if you keep adding more salt to the solution, the salt takes a longer time to dissolve, until such a time that some salt remains undissolved. Usually, there is a limit to how much solute will dissolve in a given amount of solvent. When no more solute appears to dissolve under the existing temperature and pressure, the solution is said to be saturated. In this condition, some of the solid is still dissolving in the solvent, while some of the dissolved solute is crystallizing. The rate of dissolving is equal to the rate of crystallization. This system is said to be in a state of dynamic equilibrium. For example, 37g of NaCl in 100g of water is a saturated solution at room temperature.

Any solution that contains less solute than a saturated solution is unsaturated. A solution that contains a greater amount of solute than a saturated solution is said to be supersaturated. This solution can be prepared by saturating a solution at a higher temperature and then carefully cooling it to a temperature where the solute is less soluble. Supersaturated solutions are very unstable conditions. Adding a small crystal of the solute (a seed crystal), or shaking the container causes crystallization to occur rapidly, leaving a saturated solution. The process of inducing crystallization to occur by adding a crystal to a supersaturated solution is called seeding.

Factors Affecting Solubility

The extent to which one substance dissolves in another depends on the nature of the solute and the solvent, temperature, and for gases, pressure.

Nature of the Solute and Solvent

Solubility of a solute in a solvent purely depends on the nature of both solute and solvent.

● A polar solute dissolves in a polar solvent.

● Solubility of a non-polar solute in a solvent is large.

● A polar solute has low solubility or insoluble in a non-polar solvent.

You probably have observed that alcohol and water are miscible while oil and water are immiscible. Sugar and table salt dissolve in water. You also learned that the solubility of a solute in a given solvent is favored when there is a similarity in their structures or in their polarities.

Ethyl alcohol has a structure similar to that of water. Both are polar. Thus, ethyl alcohol and water dissolve in each other as a result of two attractive forces. Water, which is a dipole, attracts the polar end of alcohol. In addition, hydrogen bonding can also occur between these two molecules.

Temperature

Generally, in many cases, solubility increases with the rise in temperature and decreases with the fall of temperature but it is not necessary in all cases. However, we must follow two behaviors:

In endothermic process, solubility increases with the increase in temperature and vice versa. For example: solubility of potassium nitrate increases with the increase in temperature.

In exothermic process, solubility decreases with the increase in temperature. For example: solubility of calcium oxide decreases with the increase in temperature.

Gases are more soluble in cold solvent than in hot solvent.

Pressure

The effect of pressure is observed only in the case of gases. An increase in pressure increases the solubility of a gas in a liquid. For example: carbon dioxide is filled in cold drink bottles (such as Coca Cola, Pepsi, etc.) under pressure. Pressure has almost no effect in the solubility of solids and liquids, but has a strong effect on the solubility of gases.

Colligative Properties of Solutions

Some physical properties of solutions are qualitatively similar to but are quantitatively different from those of pure solvents. A group of properties that depend on the amount of dissolved solute (concentration) and not on the kind or chemical nature of the solute are called colligative properties. Colligative means “depending upon the collection”. It is used because a collection of solute particles is responsible for the observed effects. The colligative properties are vapor pressure reduction, boiling point elevation, freezing point depression, and osmotic pressure.

Vapor Pressure Reduction

As vaporization continues, the number of vapor particles above the liquid increases until the rate of vaporization equals the rate of condensation and dynamic equilibrium is attained. The pressure exerted by the vapor particles on the liquid at equilibrium is known as the vapor pressure of the liquid at this temperature.

Boiling Point Elevation

The boiling point elevation of a solution is directly proportional to the number of solute particles. For dilute solutions, the boiling point elevation is proportional to molality.

DT_b = K_bc_m

where K_b is called the boiling-point-elevation constant.

Freezing Point Depression

The lower vapor pressure of a solution compared to that of a pure solvent also affects the freezing point of the solution. The freezing point of a solution is the temperature at which the first crystals of a pure solvent begin to form in equilibrium with the solutions. The freezing point of a solution is lower than that of a pure solvent. Like the boiling point elevation, the freezing point of a solution is directly proportional to the molal concentration of the solution.

ΔT_f = K_fc_m

where K_f is called the freezing-point-depression constant.

A pleasant application of the freezing point depression is in the making of homemade ice cream. The ice cream mix is put into a metal container which is surrounded by crushed ice. Then salt is put on the ice to lower its melting point. The melting of the solution tends to lower the equilibrium temperature of the ice/water solution to the melting point of the solution. This gives a temperature gradient across the metal container into the saltwater-ice solution which is lower than 0°C. The heat transfer out of the ice cream mix allows it to freeze.

Osmotic Pressure

The fourth colligative property is osmotic pressure. Recall an experiment in biology that demonstrates osmosis, the passage of solvent molecules but not of solute particles through a semi permeable membrane.

Hope that this blog helped you somehow.

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