What is meant by the relative atomic mass (Ar) of an element
It is the average mass of the isotopes of an element compared to the mass of an atom of carbon-12.
How do you calculate the relative atomic mass (Ar) if you are given the percentage abundance of two isotopes
(mass isotope 1 x abundance) + (mass isotope 2 x abundance) / 100
How do you calculate the relative formula mass (Mr) of a compound?
Add together the relative atomic masses (Ar) of all the atoms shown in the chemical formula.
What is the formula for calculating the percentage by mass of an element in a compound?
(Total Ar of the element in a compound) / (Mr of the compound) x 100
Define a mole
One mole is the relative atomic (or formula) mass of a substance in grams. The number used is 6.022x10^23
What is the equation linking moles, mass, and relative atomic/formula mass?
The Law of Conservation of Mass
Mass cannot be created or destroyed. The total mass of the reactants must always equal the total mass of the products.
What does a balanced symbol equation tell you about the moles of substances?
It shows the ratio of the number of moles of each reactant and product involved in the reaction. For example, H2 + Cl2 -> 2HCl shows that 1 mole of hydrogen reacts with 1 mole of chlorine to produce 2 moles of hydrogen chloride
What does the 2 mean in front of the NaOH
2NaOH + Cl2 –> NaOCl + NaCl + H2
It is a multiplier that indicates 2 moles of sodium hydroxide are needed for every 1 mole of chlorine gas to balance the equation.
How do you calculate the mass of a missing product or reactant in a chemical reaction?
Use the Law of Conservation of Mass, which states that the total mass of reactants = total mass of products.
For example, if 8.5g of sodium decomposes into 6.9g of sodium nitrate oxygen and oxygen, the mass of oxygen is 8.5g - 6.9g = 1.6g
What is a “limiting reactant” and why is it important?
The limiting reactant is the reactant that is completely used up first in a reaction. It is important because it determines the maximum amount of product that can be formed. Once it’s gone, the reaction stops.
What does it mean if a reactant is “in excess”?
A reactant is in excess if there is more of it than needed to react with the limiting reactant. Some of this substance will remain unreacted at the end of the experiment.
How do you determine which reactant is limiting if you have the masses of both?
- Convert both masses to moles.
- Check the molar ratio from the balanced equation.
- Compare what you have to what you need. The reactant you have “less than required” of is the limiting reactant.
How do you calculate the maximum mass of a product?
Identify the limiting reactant.
Use the molar ratio to find the moles of product formed from the moles of that limiting reactant.
Convert the moles of product back to mass using mass = moles x Mr
