C3 - Quantitative Chemistry Flashcards
Compounds all have a relative formula mass. What is this?
The relative atomic masses of all the atoms in teh molecule formula added together
Which number is the relative formula mass?
The top number
Ar of Mg = 24, Ar of Cl = 35.5
Calculate the relative formula mass of MgCl2
95 (24 + 35.5x2)
What is the formula for percentage mass of an element in a compound?
Relative atomic mass x number of atoms in that one element
——————————————————————————x100
Relative formula mass of the total compound
A mixture contains 20% of iron ions by mass. What mass of iron chloride (FeCl2) would you need to provide the iron ions in 50g of the mixture?
Ar of Fe = 56, Ar of Cl = 35.5
1) Find the mass of iron in the mixture
-The mixture contains 20% iron by mass, so in 50g there will be 50 x 200/100 = 10g of iron
2) Calculate the percentage mass of iron in iron chloride
-Percentage mass of iron - 56/ 56+(2x35.5) = 44.09…%
3) Calculate the mass of iron chloride that contains 10g of iron
-Iron chloride contains 44.09…% iron by mass, so there will be 10g of iron in 10/ 44.09/100 = 23g
SO you need 23g of iron chloride to provide the iron in 50g of the mixture
How much is a mole? What is this number known as?
6.02 x 10^23
The Avogadro constant
One mole of any substance is just what?
An amount of that substance that contains 6.02 x 10^23 particles - these atoms could be atoms, molecules, ions or electrons
The mass of the number of atoms or molecules of any substance is exactly the same number of grams as the relative atomic mass (Ar) or relative formula mass (Mr) of the element or compound. True or false?
True - in other words, one mole of atoms or molecules of any substance will have a mass in grams equal to the Ar or Mr of that substance. Some examples:
Carbon has an Ar of 12. So one mole of carbon weighs exactly 12g
Carbon Dioxide, CO2, has an Mr of 44. So one mole of CO2 weighs exactly 44g
What is the formula for the amount of moles in a substance?
Number of moles = Mass in g (of an element or compound) / Mr (of the element or compound)
How many moles are there in 66g of carbon dioxide?
1) Calculate the Mr of CO2
- 12 + (16 x 2) = 44
2) Use the formula to find out how many moles there are
- Mass/ Mr —- 66/ 44 = 1.5 mol
What mass of carbon is there in 4 moles of CO2?
There are 4 moles of carbon in 4 moles of CO2
So the mass in 4 moles of carbon = 4 x 12 = 48g
In a chemical reaction, what happens to mass?
Mass is always conserved
If there is a change in mass during an experiment, what happened?
IF THE MASS INCREASES - Likely because one of the reactants is a gas found in air, and all the products of the reaction aren’t gaseous, so all the products are detected by the scale, including the new products from the new gas
IF THE MASS DECREASES - One of the products is a gas, so part f the mass is not weighed as it is now part of the air
What is the method for balancing equations using reacting masses?
For if you know the masses of the reactants and products in the reactions
1) Divide the mass of each substance by its relative formula mass to find the number of moles
2) Divide the number of moles of each substance by the smallest number of moles in the reaction
3) If any of the numbers aren’t whole numbers, multiply all the numbers by the same amount so that they all become whole numbers
4) Write out the balanced equation for the reaction by putting these numbers in front of the chemical formulas
Why will a reaction stop? What is that called
When one reactant is used up. That reactant is called a limiting reactant
