Quantitative Chemistry Flashcards
What is the “relative atomic mass” of an atom?
The average mass of an atom, taking into account the mass and amount of each isotope.
What is the “relative formula mass”?
The sum of all the relative atomic masses shown in a formula
E.g.: 2H + O2 –> H2O = 18.
What is a “mole”?
What do we call the value of a mole?
A mole is 6.02 x 10^23 particles of a substance.
This number is known as “avagadro’s constant”
What is the mass of one male equal to?
The mass of one mole is equal to the relative formula mass in grams
E.g.: 1 mole of carbon atoms = 12g
Why must chemical equations be “balanced”?
When the number of atoms on each side of the equation are equal. This is because atoms cannot be created or destroyed during reactions.
What is the law of “conservation of mass”?
The total mass of reactants and total mass of products must be equal. This is because the same amount of atoms are present at the start and end of a reaction, as atoms cannot be created or destroyed in chemical reactions.
When may the mass of the products be higher or lower in a reaction?
Higher = when metals react with oxygen in the air. So the extra mass is the mass of the oxygen that has reacted with the metal to form a metal oxide
Lower = During thermal decomposition reactions (when a substance breaks down into a smaller one), some of the products escape the container as a gas, so the products have a lower mass.
How do you calculate the amount of moles of a substance?
Moles = Mass/Molecular Mass
Calculate the number of moles in one gram of magnesium
Mass = 1g
Molecular mass = 24.
1/24 = 0.04 moles.
How do you calculate the balancing numbers in an equation from the reacting masses?
1) Calculate the moles of each substance
2) Find the simplest whole number ratio of these mole values by dividing all mole values by the smallest one
3) Turn into whole numbers if they are not already
1.2g of magnesium (Mg) reacts with 0.8g oxygen (O2) to make 2.0g of MgO. Write the full balanced equation for the reaction.
1) Work out moles (1.2/24 = 0.05, 0.8/32 = 0.025, 2.0/40 = 0.05)
2) Divide by smallest mole number. 0.05/0.025 = 2
3) Ratio is 2:1:2
Therefore: 2Mg + O2 –> 2MgO
Why may the mass of the products of a reaction be less than the original reactants?
1) Some reactions do not go to completion - i.e.: because they are reversible
2) Some of the product is lost, e.g.: it is left on the apparatus
3) Some of the reactants may react in different ways to the desired reaction
What is the “yield” of a reaction?
How do you calculate % yield?
Yield = the amount of product made in a reaction
% Yield = 100 x (Mass of the product actually made/Maximum theoretical mass of a product)
In a reaction to manafacture paracetamol, the maximum theoretical mass of the product was 200g, but the actual yield was 150g. Calculate the % yield.
Give 2 reasons why the percentage was less than 100%
100 x (150/200) = 75%
1) The reaction may not have gone to completion
2) Some of the product is lost, i.e.: it remains in the apparatus
