C4. Chemical Calculations Flashcards
What is the law of conservation of mass?
The law of conservation of mass states that no atoms are lost or made during a chemical reaction so the mass of the products equals the mass of the reactants
Write a balanced equation of magnesium reacting with hydrochloric acid.
Mg(s) + 2HCl (aq) –> MgCl₂ (aq) + H₂ (g)
Define relative atomic mass and relative formula mass
RAM - average mass of atoms in an element taking into account masses and abundance of its isotopes, relative to ¹²C
RFM- sum of RAM’s of all atoms in the formula
What is the relative formula mass of:
A) CaF₂?
B) C₂H₁₂O₆?
CaF₂ - (Ar values: Ca= 40, F = 19)
40 + 19 + 19 = 78
C₂H₁₂O₆- (Ar values: C=12, H=1, O=16)
(12×6)+(1×12)+(16×6) = 180
The following reaction occurs in a test tube under a Bunsen Burner:
4 MgO (s) + CH₄ (g) –> 4 Mg (s) + 2H₂O (g) + CO₂ (g)
The carbon dioxide and water escape from the test tube.
Use the equation to explain why.
They are both gases
What is Avogadro’s constant?
The number of atoms, molecules or ions in a mole of a given substance. The value of the constant is 6.02×10²³
What is the formula that links mass, molecular mass and moles together?
Mass = Mr × Moles
What is the mass of 20 moles of calcium carbonate, CaCO₃?
Mass = Mr x Moles
Mr=100
100 × 20 = 2000g
Calculate the amount of carbon dioxide in moles in 0.32 g of carbon dioxide?
Relative atomic masses (Ar): carbon = 12, oxygen = 16
Moles= Mass / Mr
0.32 / 44 = 0.007
Nitrogen and hydrogen form ammonia shown by the following equation:
N₂ + 3 H₂ ⇌ 2 NH₃
Calculate the mass of nitrogen needed to form 6.8 tonnes of ammonia
Relative atomic mass (Ar): H=1; N=14
Step 1- Work out the number of moles of ammonia (Mr of ammonia= 17)
6800000/17 = 400000 moles of ammonia
Step 2- Use the balanced equation and number of moles of ammonia to work out the number of moles of nitrogen
The ratio of nitrogen to ammonia is 1:2
Therefore the number of moles of nitrogen is 400000/2= 200000
Step 3 - Work out the mass of nitrogen (Mr of N₂ is 28)
200000 × 28= 5600000 = 5.6tonnes
State what we mean by a limiting reactant in a chemical reaction?
In a chemical reaction involving two reactants, it is common to use an excess of one of the reactants to ensure that all of the other reactant is used. The reactant that is completely used up is called the limiting reactant because it limits the amount of products
How much oxygen will be given off from 40.8g of hydrogen peroxide?
Step 1: Write the balanced equation 2H₂O₂ –> 2 H₂O + O₂ Mr of H₂O₂ = 34
Step 2: Number of moles in 40.8g : 40.8/34 = 1.2 moles
Ratio in the balanced equation of H₂O₂ : O₂ = 2:1
Step 3: Therefore number of moles of O₂ = 0.6 moles
Step 4: Mass of oxygen = 0.6 × 32 (Mr of O₂) = 19.2
What is the two formulae that link concentration, mole/mass and volume together?
Concentration (g per dm³) = Mass (g)/ Volume (dm³)
Concentration (mole per dm³) = nr of moles/volume (dm³)
31.0 cm³ of potassium hydroxide solution neutralised 25.0cm³ of 2.0 moldm-³ nitric acid
HNO₃ + KOH –> KNO₃ + H₂O
Calculate the concentration of the potassium hydroxide solution in moldm-³
Step 1: Calculate the moles of HNO₃ used = Concentration × volume
2 × 0.025dm³ (25/1000 to convert the units) = 0.05 moles
Step 2: Calculate the moles of KOH
Ratio is 1:1 therefore numbers of moles of KOH = 0.05
Step 3: Calculate the concentration of KKH
Volume = Moles/concentration; 0.05/ 0.031 = 1.61
What is the molar volume of a gas at room temperature and pressure?
1 mole of a gas at room temperature and pressure occupies 24dm³
What is titration?
A technique for finding the concentration of a solution by reacting a known volume of this solution of a known concentration
How do you conduct a titration?
a) Rinse the pipette with a solution of unknown concentration. Use the pipette to measure out the known volume of this solution
b) Add an indicator (a substance that changes colour at the end of titration)
c) Rinse the burette with a solution of known concentration. Discard the liquid. Use a burette to gradually add the solution of a known concentration
d) When indicator changes colour (at the end point), the volume added is recorded
e) It is important to get concordant volume results - they have to lie close to each other
f) Suitable calculations are performed to find the concentration
Why is not always possible to obtain the theoretical amount of product in a chemical reaction?
The reaction may not go to completion because it 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 (side reactions may occur)
How is the percentage yield of a product in a chemical reaction?
% Yield = Actual mass of a product × 100% ÷ Maximum theoretical mass of product
What is the % yield of NH₃ is 40.5g NH₃ is produced from 20.0 mol H₂ and excess N₂?
Step 1 - Write the balanced equation
N₂ + 3H₂ –> 2 NH₃
Step 2- Calculate the theoretical amount of NH₃, Moles MH₃ (ratio of H₂ to NH₃ is 3:2); of 20/1.5 = 13.3 moles
13.3 × 17 (Mr of NH₃) = 227
Step 3- Calculate percentage yield of NH₃
40.5/227= 17.8%
What is atom economy?
A measure of the amount of starting materials that end up as useful products.
It is a ratio of the relative formula mass of desired product to the sum of relative formula masses of reactants
Look at the equations for the two reactions that produce CuCl₂
Reaction I: CuCO₃ + 2 HCl –> CuCl₂ + H₂O + CO₂
Reaction II: CuO + 2 HCl –> CuCl + H₂O
Reactive formula masses: CuO =79.5; HCl = 36.5; CuCl₂ = 134.5; H₂O = 18
Which reaction has a better atom economy?
Reaction II (look at the reactants):
Total formula mass of the reactants = 152.5
Formula mass of CuCl₂ = 134.5
(134.5/152.5) × 100% = 88.2%
