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General Chemistry - Solutions

Think about dissolving sugar in a glass of water, because the water dissolves the sugar, it’s called, the solvent. And, because the sugar is dissolved in the water it is called the solute. Sugar water is an aqueous solution which means that the solution is water. A relatively weak solution is called dilute, and a relatively strong solution is called concentrated. 

Measuring Concentrations 
The most commonly used unit for concentration is called molarityM It gives the number of moles of solute dissolved per liter of solution. 

So, molarity M = Moles of Solute / Liters of Solution 

Molality is another unit for measuring concentration and its the number of moles of solute per kilogram of solvent. 

So, molality m = Moles of Solute / Mass (Kg) of Solvent 

Solubility and Saturation :
If you take a glass of water and keep adding tablespoon after tablespoon of sugar, you will reach a point at which the water refuses to dissolve any additional sugar. At that point, we say that the solution is saturated. 

Boiling Point Elevation, Freezing Point Depression and Vapor Pressure 

When a solid solute dissolves in a liquid solvent, it raises the solvent’s boiling point and lowers its freezing point. 

Rule number 15: 

The change in boiling point or freezing point is always equal to sum constant K times the molality of the solution times the number of dissolved particles produced by each molecule of solute or ΔT = KMX. 

If we are dealing with boiling point the constant is called the boiling point elevation constant. If we are dealing with freezing point the constant is called the freezing point depression constant. Boiling point elevation constants and freezing point depression constants vary with the solvent. Each solvent has its own. 

Vapor Pressure :
Vapor pressure is the pressure from the gas that is evaporating from the exposed surface of a liquid. If we were to add some solute to a liquid, its vapor pressure would decrease according to the amount of solute we add. A liquid that has a high vapor pressure is said to be volatile. 

Let us summarize what we have just said. When you add non-volatile substances to a solution – one, the vapor pressure is lowered. Two, the boiling point is raised. And three, the freezing point is lowered.