Separate Chemistry - 5.3 Flashcards
What does the law of the conservation of mass state?
No atoms are lost or made during a chemical reaction
A piece of magnesium is added to acid. What would happen to the mass reading on the balance and why?
It would stay the same because there is a lid on the conical flask so no gas can escape.
Count the number of elements in the following compounds:
a) CaO
b) Al2O3
c) H2SO4
a) CaO - 2 elements (Ca and O)
b) Al2O3 - 2 elements (Al and O)
c) H2SO4 - 3 elements (H, S and O)
Count the number of atoms in the following compounds:
a) CaO
b) Al2O3
c) H2SO4
a) CaO - 2 atoms (1 x Ca and 1 x O)
b) Al2O3 - 5 atoms (2 x Al and 3 x O)
c) H2SO4 - 7 atoms (2 x H, 1 x S and 4 x O)
Calculate the relative formula mass (Mr) of each of the compounds (a, b & c) below.
AR: Mg = 24; Cl = 35.5; Ca = 40; O = 16; Al = 27
a) CaO
b) MgCl2
c) Al2O3
*this is just an example, you need to be able to do this for any compound.
a) CaO: 40 + 16 = 56
b) MgCl2: 24 + (35.5x2) = 95
c) Al2O3: (27x 2) + (16 x 3) = 102
A piece of magnesium is added to acid. What would happen to the mass reading on the balance and why?
It would decrease because there is no lid on the conical flask so gas can escape and therefore mass is ’lost’ from the flask.
Calculate the uncertainty in the following sets of results
a) 1, 3, 6, 7, 10
b) 22, 25, 26, 25, 24
*this is just an example, you need to be able to do this for any set of data given.
Use the equation: uncertainty = range / 2
a) 1, 3, 6, 7, 10
Range = 10 – 1 = 9. Uncertainty = 9/2
Uncertainty = ± 4.5
b) 22, 25, 26, 25, 24
Range = 26 – 22 = 4. Uncertainty = 4/2
Uncertainty = ± 2
Calculate the percentage mass of hydrogen in:
Ar: N = 14 ; H = 1 ; S = 32 ; O = 16.
a) NH3
b) H2SO4
Give your answer to the nearest whole number.
*this is just an example, you need to be able to do this for any compound when given the appropriate information.
% mass = (Ar x number of atoms of the element) / (Mr of compound) x 100
a) NH3 : (3x1) / 17 = 0.176
0. 176 x 100 = 18 %
b) H2SO4 : (2x1) / 98
0. 02 x 100 = 2 %
Higher Q. How many molecules, atoms or ions are in one mole of a substance?
6.02 x 1023
Higher Q. How many moles are in the following?
a) 46 g of CaO?
b) 50 g of O2?
Number of moles = (mass in g)/Mr
a) Mr of CaO = 40 + 16 = 56.
Number of moles = 46 g /56 = 0.82 moles
b) Mr of O2 =16
Number of moles = 50g / 32 = 1.6 moles
Higher Q. Describe the equations below in words in terms of the number of moles of each substance reacting.
1) 2Mg + O2 → 2MgO
2) H2SO4 + 2NaOH → Na2SO4 + 2H2O
1) 2 moles of magnesium react with one mole of oxygen to produce two moles of magnesium oxide.
2) One mole of sulfuric acid reacts with two moles of sodium hydroxide to produce one mole of sodium sulfate and two moles of water.
Higher Q. Calculate the mass of magnesium needed to produce 4.6 tonnes of magnesium oxide.
2Mg + O2 → 2MgO
*this is just an example, you need to be able to do this for any equation when given a mass of a reactant or product.
Step 1 – Make sure the equation is balanced - This equation is already balanced.
Step 2 – Calculate the Mr of 2Mg and the Mr of 2MgO.
Mr of 2Mg = 48, Mr of 2MgO =80
Step 3 – You now know 48 tonnes of magnesium would produce 80 tonnes of magnesium oxide.
Step 4 – Find how much magnesium you would need to produce 1 tonne of magnesium oxide: (48/80) magnesium and (80/80) magnesium oxide.
Therefore 0.6 tonnes of magnesium would produce 1 tonne of magnesium oxide.
Step 5 – Multiply both sides by 4.6 to work out how much magnesium would be needed to make 4.6 tonnes of magnesium oxide.
(0.6 x 4.6) = 2.76 tonnes
Higher Q. Calculate the mass of CO2 produced when 50 g of CaCO3 is thermally decomposed.
CaCO3 → CaO + CO2
*this is just an example, you need to be able to do this for any equation when given a mass of a reactant or product.
Step 1 – Make sure the equation is balanced - This equation is already balanced.
Step 2 – Calculate the Mr of CaCO3 and the Mr of CO2.
Mr of CaCO3 = 100, Mr of CO2 = 44
Step 3 – You now know 100 g of CaCO3 would produce 44 g of CO2.
Step 4 – Find what 1 g of CaCO3 would produce.
(100/100) CaCO3 and 44/100 CO2
1g of CaCO3 would produce 0.44 g of CO2
Step 5 – Multiply both sides by 50 to work out what 50 g of CaCO3 would make.
50 g of CaCO3 would make (0.44 x 50g) of CO2
50 g of CaCO3 would make 22g of CO2
Higher Q. Write a balanced symbol equation for the following reaction:
48 g of magnesium (Mg) reacts with 32 g oxygen (O2) to produce 80 g of magnesium oxide (MgO).
*this is just an example, you need to be able to do this for any equation when given the appropriate information.
Find ratio of moles in equation by finding number of moles of each substance using the equation:
Number of moles=(mass in g)/Mr
Mg: 48/24 = 2 ; O2: 32/32 =1 ; MgO: 80/40 = 2
Ratio = 2Mg: 1O2 : 2MgO
Balanced equation: 2Mg + O2 → 2MgO
Higher Q. What is a ‘limiting reactant’?
The first reactant that gets used up in a reaction and causes the reaction to stop.
Higher Q. Why is one reactant often added in excess to a reaction?
To ensure that all of the other reactant is used up.
Higher Q. What are the two possible units for concentration?
g/dm3 or mol/dm3
Higher Q. What equation links concentration, mass of solute and volume of solvent?
Concentration =(mass of solute)/(volume of solvent)
Higher Q. What equation links concentration, number of moles of solute and volume of solvent?
Concentration =(number of moles of solute)/(volume of solvent)
Higher Q. What is the mass of one mole of a substance, in grams, equal to?
The relative formula mass of the substance
Higher Q. Calculate the concentration of sodium hydroxide solution when 44 g of sodium hydroxide is dissolved in 0.5 dm3 of water.
*this is just an example, you need to be able to do this for any equation when given a mass of a reactant or product.
44 g / 0.5 dm3
= 88 g / dm3
Higher Q. Calculate the concentration of sodium hydroxide solution when 3.5 moles of sodium hydroxide is dissolved in 100 cm3 of water. Give your answer in mol/dm3.
*this is just an example, you need to be able to do this for any equation when given a mass of a reactant or product.
Convert volume of water to dm3
100 cm3 / 1000 = 0.1 dm3
3.5 moles / 0.1 dm3 = 35 mol/dm3
Higher Q. What is the value of the Avogadro Constant?
6.02 x 1023 per mole
Separate Q. Give three reasons why you might not achieve the amount of product you have calculated (theoretical yield).
- The reaction may not go to completion because it is reversible.
- Some of the product may be lost when it is separated from the reaction mixture.
- Some of the reactants may react in ways different to the expected reaction.
Separate Q. What does ‘yield’ mean?
The amount of product that is obtained
Separate Q. What equation would you use to work out the percentage yield of a reaction?
% yield=(actual yield)/(theoretical yield) x 100
Separate Higher Q. What volume of carbon dioxide is formed when 20 dm3 of carbon monoxide reacts with oxygen?
2CO (g) + O2 (g) → 2CO2 (g)
2CO (g) + O2 (g) → 2CO2 (g)
Ratio of moles of carbon monoxide to carbon dioxide is 2:2 or 1:1.
Therefore 20 dm3 of carbon monoxide will produce…
20 dm3 of carbon dioxide
Separate Higher Q. What’s the volume of the following gases at room temperature and pressure?
a) 24.5 g of Oxygen
b) 53.6 g of Chlorine
*these are just examples, you need to be able to do this for any gas.
Volume of gas= (Mass of gas) / (Mr of gas) x 24
a) Oxygen: Volume of gas = 24.5/32 x 24 = 18.4 dm3
b) Chlorine: Volume of gas= 53.6/71 x 24 = 18.1 dm3
Separate Q. What does atom economy mean and why is it important?
- Atom economy is a measure of the amount of starting materials that end up as useful products.
- It’s important for sustainable development and economical reasons.
Separate Q. How would you calculate percentage atom economy?
% atom economy = (Mr of desired product) / (Sum Mr of all reactants) x 100
Separate Higher Q. If one mole of oxygen occupies 24 dm3 at room temperature, how much volume would Nitrogen occupy?
• 24 dm3
*one mole of any gas at 20 oC and 1 atm of pressure occupies a volume of 24 dm3
