Stiochiometry

In balanced equations, the coefficients represents the number of moles. To find the number of moles of a compound or element in a balanced equation, we divide the given mole by the coefficient.

Example - : If 0.20 mol of methane reacts with oxygen, how many moles of each product are produced?

- first write a balanced equation, be sure that the equation is correctly balanced:
2 CH4 + 4O2 -> 2 CO2 + 4 H2O
- to find the number of moles of CO2: multiply the number of moles of methane by CO2's coefficient which is 2, and divide by the number of moles of methane.
(0.20 mol CH4 x 2 mol CO2) / (2 mol CH4) = 0.4 mol CO2

When converting moles to mass, one more step is needed (g/mol)

Example - : How many grams of water are produced if 1.0 mol of phosphoric acid is neutralized by barium hydroxide?

- first write a balanced equation:
2 H3PO4 + 3 Ba(OH)2 -> Ba(PO4) + 6 H2O
- to find the number of moles of H2O: multiply the number of moles of H3PO4 (phosphoric acid) by H2O's coefficient which is 2, and divide the number of moles of H3PO4
(1.0 mol H3PO4 x 6 H2O) / (2 mol H3PO4) = 3.0 mol H2O
- now multiply H2O's molar mass and divide by mol (g/mol)
(3.0 mol x 18 g) / (1 mol) = 54 g H20

Here's a video of Chemguy solving many moles of silver chloride forms when 2.6 mol of KCl reacts with excess silver nitrate in solution:


That is all. Mr. Doktor is the best!

-- Jael Lumba

Classifying Chemical Reactions

There are four main types of chemical reactions. They are direction combination, decomposition, single-replacement, and double-replacement.

Direct Combination Reactions
A + B -> AB
EXAMPLE: 2 Na + Cl2 -> 2 NaCl

- Combustion reactions are direct combination when the products are carbon dioxide and water:
C2H5OH + 3 O2 -> 2 CO2 + 3 H2O

Decomposition Reactions
AB -> A + B
EXAMPLE: 2 H20 -> 2 H2 + O2

Single-Replacement Reactions
A + BX -> AX + B
EXAMPLE: Mg + CuSO4 -> Mg SO4 + Cu

Double-Replacement Reactions
AX + BY -> AY + BX
EXAMPLE: CaCO2 + 2 HCl -> CaCl2 + H2CO3

That is all. Mr. Doktor is the best!

-- done by Jael Lumba

Concentration, number 9!

We learned that a solution is a homogeneous mixture. In a solution, there is a solute and solvent. A solute is the component present in smaller amount, a solvent is the component present in a smaller amount. Concentration (con'c) is amount of solute / amount of solvent.

The possible units for concentration are g/mL, g/L, mg/L, % by mass, % by volume, etc..
- mol / L = molarity

Concentration is found by using C = n / v
- C = concentration
- n = number of moles
- v = volume

- Example -: Bob dissolves 38.0 g of NaOH in enough water to make 200.0 mL of solution. Find the molarity.
- [NaOH] = ? - first find the molar mass which is 40.0g
- find the amount of moles by dividing the mass of NaOH by its molar mass: 40.0g / 38.0g = 1.05 mol
- find the molarity in mol/L: (1.05 mol x 1000mL) / (200 mL x 1 L) = 5.25 mol / L

That is all. Mr. Doktor is the best!

-- done by Jael Lumba