Jan.11--Molar Volume of a Gas at STP

Gases expand and contract (change volume) with changes in temperature and pressure.


STP = Standard Temperature and Pressure = a standard condition to compare volume of gases

STP = 1 atmosphere of pressure and a temperature of 0 ℃

At STP, 1 mole of gas occupies 22.4 L

Conversion Factors:

22.4 L of gas / 1 mole of gas  OR  1 mole of gas / 22.4 L of gas

Ex. Calculate the volume occupied by 3.4 g of ammonia at STP.

   NOTE: Ammonia = NH₃

     NH₃ = 14.0 + 1.0 x 3 = 17.0 g / mol

3.4 g x (1 mole / 17.0 g) = 0.2 moles

(22.4 L / 1 mole) x 0.2 mol = 4.5 L

Jan 7--Diluting Solutions

Key Idea: The moles of solute before and after diluting is constant.

Moles solute before = Moles solute after

M1 L1 = M2 L2

For example:

A concentrated HCl is 24.5 mole/L. How would you make up 125 mL of 2.53 mole/L of HCl?

 1. Solve for how much volume first.(L1)

 2. From the fomula : M1L1 = M2L2

 3. 24.5 mole/L x L1 = 125 mL x 2.53 mole/L
             L1= (125 mL x 2.53 mole/L) / 24.5 mole/L
             L1= 12.9 L

"Molarity" of Solutions (Molar Concentration)

Solute: the chemical that is in the smaller quantity (being dissolved)

Solvent: the chemical that is in the larger quantity (does the dissolving)

What is the concentration of a solution?

- It represents the amount of solute dissolved in a certain volume of the solution.

Molar Concentration / Molarity: the number of moles of solute in one litre of a solution.

Unit: moles / L

Formulae: Molarity = moles of solute (mol) / volume of solution (L)

      or M = mol / L