C3 Quantitative Chemistry 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) → MgC2 (aq) + H2 (g)
Define relative atomic mass and relative formula mass.
RAM - average mas of atoms in an element taking into account masses and abundance of its isotopes, relative to 12C.
RFM - sum of RAMs’ of al atoms ni the formula.
What is the relative formula mass of:
A) CaF2
B) C6H12O6
CaF2 -(Ar values: Ca =40, F= 19) 40 + 19 + 19 = 78
CH12O6 -(Ar values: C=12, H=1,0=16)
(12 x 6) + (1 x 12) + (16 x 6) = 180
The following reaction occurs in a test tube under a Bunsen Burner:
4MgO (s) + CH4 (g) → 4Mg (s) + 2H20 (g) + CO2 (g)
The carbon dioxide and water escape from the test tube.
Use the equation to explain why.
They are both gases
Suggest how you could increase the precision
Measure to more decimal places or use a more sensitive balance / apparatus
What is Avogadro’s constant?
The number of atoms, molecules or ions in a mole of a given substance. The value of the constant si 6.02 x10^23.
What is the formula that links mass, molecular mass and moles together
Mass = Mr x Moles
Write the equation that links Mass, Mr and Moles
Mass = Mr x Moles
Mr =100
100 × 20 =2000 g
What si the mas of 20 moles of calcium carbonate, CaCO3
Mass = Mr x Moles
Mr =100
100 × 20 = 2000 g
Calculate the amount of carbon dioxide in moles in 0.32 of carbon dioxide.
Relative atomic masses (Ar): carbon = 12, oxygen = 61
Moles = Mass / Mr
0.32 / 4 = 0.007
Nitrogen and hydrogen form ammonia shown by the following equation:
N2 (g) +3H2 (g) ⇌ 2 NH3 (g)
Calculate the mass of nitrogen needed ot form 6.8 tonnes of ammonia.
Relative atomic masses (Ar): H = 1; N = 14
Step 1- Work out the number of 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 ot 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 mas of nitrogen M(r of N, si 28)
200000 x 28 = 5600000 g = 5.6 tonnes.
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 ti limits the amount of products.
How much oxygen will be given of from 40.8 g of hydrogen peroxide?
Step 1: Write the balanced equation 2 H20 2(l)) →2 H202 = 34
Step 2: Number of moles in 40.8 g: 40.8/34 = 1.2 moles
Ratio in the balanced equation of H202 : 02 = 2:1
Step 3: Therefore number of moles of 0, = 0.6 moles
Step 4: Mass of oxygen = 0.6 × 32 ( Mr of 02) = 19.2
Write down the two formulae that link concentration, mole/mass and volume together.
Concentration (g per dm3) = Mass (g)/Volume (dm3)
Concentration (mol per dm3) = no of moles/volume (dm3)
31.0 cm3 of potassium hydroxide solution neutralised 25.0 cm of 2.0 moldm-3 nitric acid.
HNO3 + KOH → KNO + H20
Calculate the concentration of the potassium hydroxide solution in moldm-3
Step 1: Calculate the moles of HNO3 used = Concentration x volume
2 x 0.025 dm (25/1000 to convert the units) = 0.05 moles
Step 2: Calculate the moles of KOH
Ratio is 1:1 therefore number of moles of KOH = 0.05
Step 3: Calculate the concentration of KOH
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 agas at rom temperature and pressure occupies 24 dm3
What is titration?
A technique for finding the concentration of a solution by reacting a known volume of this solution with a solution of known concentration.
How do you conduct a titration?
1) Rinse the pipette with a solution of unknown concentration. Use the pipette to measure out the known volume of this solution.
2) Ad an indicator (a substance that changes colour at the end of titration)
3) Rinse the burette with a solution of known concentration. Discard the liquid. Use a burette to gradually add the solution of a known concentration.
4) When indicator changes colour (at the end point), the volume added is recorded
5) It is important to get concordant volume results - they have to lie close to each other
6) Suitable calculations are performed to find the concentration.
Why is it not always possible to obtain the theoretical amount of product in a chemical reaction?
- 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 (side reactions may occur).
How is the percentage yield of a product in a chemical reaction?
%Yield = Actual mas of a product x 100% / Maximum theoretical mass of product
What si the % yield of NH, fi 40.5 g NH3 is produced from 20.0 mol H2 and excess N2?
Step 1 - Write the balanced equation
N2 +3H2 →2NHg
Step 2 - Calculate the theoretical amount of NH, Moles NH (ratio of H2 to NH3 is 3:2); of 20/1.5 = 13.3 moles
13.3 X 17 (Mr of NH3) = 227
Step 3 - Calculate percentage yield of NH3
40.5/227 × 100 = 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 (s) + 2 HCL (aq) + H20 (l) + CO2 (g)
Reaction II: CuO (s) +2HCl (aq) → CuCl2 (aq) +H2O (l)
Reactive formula masses: CuO = 79.5; HCI = 36.5; CuCI, = 134.5;
H20 = 18
Which reaction has a better atom economy?
Reaction II (look at the reactants):
Total formula mass of reactants = 152.5
Formula mas of CuCl2 = 134.5 (134.5/152.5) x100% = 88.2%
Avogadro constant:
The number of atoms, molecules or ions in a mole of a given substance.
Avogadro’s law:
Equal amounts in moles of gases occupy the same volume under the same conditions of temperature and pressure.
Concentration:
The amount of substance (e.g. the mass) in a certain volume of a solution.
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.
Limiting reactant:
The reactant that is completely used up since it limits the amount of products formed.
Mole:
Chemical amounts are measured in moles. The mole is the unit for amount of substance. The symbol for the unit mole is mol.
Relative formula mass:
The sum of the relative atomic masses of the atoms in the numbers shown in the formula. It is numerically equal to the mass of one mole of a substance in grams.
Thermal decomposition:
The reaction that occurs when heat is applied to a compound causing it to break down into its different chemical constituents.
Uncertainty:
All measurements have a degree of uncertainty regardless of precision and accuracy. Uncertainty can be due to the limitations of the measuring equipment or due to the skill of the experimenter carrying out the measurements
