C3- Quantitative Chemistry Flashcards
Percentage mass of an element in a compound =
relative atomic mass X number of atoms of that element/ relative formula mass of the compound X100
What is a mole is standard form
6.02 X10^23
Number of moles=
mass in g / relative formula mass
Number of moles=
mass ÷ relative formula mass
How do you balance the equation when the number of moles is different
- Divide the mass of each substance by its relative formula mass to find the number of moles
- Divide the number of moles of each substance by the small number of moles in the reaction
- If any of the numbers aren’t whole numbers, multiply all the numbers by the same amount so that they all become whole numbers
- Write the balanced symbol equation for the reaction by putting these numbers in front of the the chemical formulas
How to calculate the mass of a product formed in a reaction
- Write out the balanced equation
- Work out relative formula masses (Mr) of the reactant and product you want
- Find out how many moles there are of the substance you know the mass of
- Use the balanced equation to work out how many moles there’ll be of the other substance. In this case, that’s how many moles of product will be made of this many moles of reactant
- use the number of moles to calculate the mass
True/False
At the same temperature and pressure, equal numbers of moles any gas will occupy the same volume
True
Volume of gas =
Mass of gas(g)/relative formula mass of gas X24
What is concentration a measure of
How crowded things are
Concentration equation using solute and solvent=
mass of solute(g)/volume of solvent dm^3
Concentration equation using number of moles solute and volume solvent=
divide moles of solutE by volume of solutioN
What is the purpose of a titration experiment
They let you find the volume needed for 2 solutions to react together completely
Define atom economy
% of reactants forming useful products
Atom economy =
Relative formula mass of desired products/ Relative formula mass of all reactants X100
How does atom economy relate to the environment
The higher the atom economy the greener it is
What are the pros for having a higher atom economy for a business
Better for profit and the environment
What is percentage yield
Actual yield compared to theoretical yield
Define yield
The amount of product you get
What is the equation for percentage yield.
Percentage yield=
mass of product actually mad(g)/maximum theoretical mass of product (g)X100
Soluble salts are formed by reacting metal oxides with acids. Give one other type of substance that can react with an acid to form a soluble salt.
- metal
- (metal) hydroxide
- (metal) carbonate
- alkali
Calcium nitrate contains the ions Ca2+ and NO3
Give the formula of calcium nitrate.
Ca(NO3)2
Describe a method to make pure, dry crystals of magnesium sulfate from a metal
oxide and a dilute acid.
- use magnesium oxide and sulfuric acid
- add sulfuric acid to a beaker
- warm sulfuric acid
- add magnesium oxide
- stir
- continue adding until magnesium oxide is in excess
- filter
- using a filter paper and funnel
- to remove excess magnesium oxide
- heat solution in an evaporating basin
- to crystallisation point
- leave to crystallise
- pat dry with filter paper
Why can alkaline batteries not be recharged?
the reaction is not reversible
Explain why graphite conducts electricity.
Answer in terms of the structure and bonding in graphite.
each carbon / atom forms 3 (covalent) bonds one electron per carbon / atom is delocalised (so) these electrons carry charge through the graphite or (so) these electrons move through the structure
Name the products formed when chlorine solution reacts with
potassium iodide solution.
potassium chloride and iodine
Chlorine reacts with hydrogen to form hydrogen chloride.
Explain why hydrogen chloride is a gas at room temperature.
Answer in terms of structure and bonding.
hydrogen chloride is made of small molecules (so hydrogen chloride) has weak intermolecular forces (intermolecular forces) require little energy to overcome
Titanium is a transition metal.
Titanium is extracted from titanium dioxide in a two stage industrial process.
Stage 1 TiO2 + 2 C + 2 Cl2 TiCl4 + 2 CO
Stage 2 TiCl4 + 4 Na Ti + 4 NaCl
Suggest one hazard associated with Stage 1.
chlorine is toxic
Titanium is a transition metal.
Titanium is extracted from titanium dioxide in a two stage industrial process.
Stage 1 TiO2 + 2 C + 2 Cl2 TiCl4 + 2 CO
Stage 2 TiCl4 + 4 Na Ti + 4 NaCl
Water must be kept away from the reaction in Stage 2.
Give one reason why it would be hazardous if water came into contact with sodium
any one from: • very exothermic reaction • produces a corrosive solution • produces hydrogen, which is explosive / flammable
Dilute hydrochloric acid is a strong acid.
Explain why an acid can be described as both strong and dilute.
(strong because) completely
ionised (in aqueous solution)
(dilute because) small amount of
acid per unit volume
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) + 2 HCl(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) B) C6H12O6
CaF2 - (Ar values: Ca = 40, F = 19) 40 + 19 + 19 = 78 --- C2H12O6 - (Ar values: C = 12, H = 1, O = 16) (12 x 6) + (1 x 12) + (16 x 6) = 180
The following reaction occurs in a test tube under a Bunsen
Burner:
4 MgO(s) + CH4
(g) → 4 Mg(s) + 2 H2O(g) + CO2
(g)
The carbon dioxide and water escape from the test tube.
Use the equation to explain why
They are both gases
Calculate the mean of magnesium produced and suggest how you
could increase the precision of the results
3.3 + 3.5 + 3.2) / 3 = 3.3
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 is 6.02 x 10.23
What is the formula that links mass, molecular mass and moles
together
Mass = Mr x Moles
What is the mass of 20 moles of calcium carbonate, CaCO3
Mass = Mr x Moles
Mr = 100
100 x 20 = 200 g
Calculate the amount of carbon dioxide in moles in 0.32 g of carbon dioxide.
Relative atomic masses (A
r
): carbon = 12, oxygen = 16
Moles = Mass / Mr
0.32 / 44 = 0.007
Nitrogen and hydrogen form ammonia shown by the following equation: N2(g) + 3 H2(g) ⇌ 2 NH3(g) Calculate the mass of nitrogen needed to form 6.8 tonnes of ammonia. Relative atomic masses (A r ): 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 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 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 it limits the amount of products
Hydrogen peroxide decomposes in water to form water and oxygen. How many grams of oxygen gas will be given off from 40.8 g of hydrogen peroxide?
Step 1: Write the balanced equation 2 H2O2(l) → 2 H2O + O2(g) Mr of H2O2 = 34Step 2: Number of moles in 40.8 g : 40.8/34 = 1.2 molesRatio 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 x 32 (Mr of O2
) = 19.2
Write down the two formulae that
link concentration, mass and
volume together
concentration (g per dm3) = Mass (g)/Volume (dm3)
Concentration (mol per dm3) = mr of moles/volume (dm3
31.0 cm3 of potassium hydroxide solution neutralised 25.0 cm3 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 x volume
2 x 0.025 dm3
(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 a gas at room 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?
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 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 mass of a product ÷
Maximum theoretical mass of product x100
What is the % yield of NH3 if 40.5 g NH3 is produced from 20.0 mol H2 and excess N2 ?
Step 1 - Write the balanced equation N2 \+ 3 H2 → 2 NH3 Step 2 - Calculate the theoretical amount of NH3 .Moles NH3 (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 x 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) → CuCl2(aq) + H2O(l) + CO2(g)
Reaction II: CuO(s) + 2 HCl(aq) → CuCl2(aq) + H2O(l)
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 reactants = 152.5 Formula mass of CuCl2 = 134.5 (134.5/152.5) x 100% = 88.2%
The hydrogen produced was collected.
Describe how to test the gas to show that it is hydrogen.
Test …………………………………………………………………………………………..
…………………………………………………………………………………………………
Result ………………………………………………………………………………………
…………………………………………………………………………………………………
lit splint or ignite the gas
(squeaky) pop / explosion
Explain why the magnesium has to be heated to start the reaction.
because it provides energy (for the reaction)
to break bonds (in the reactants)
Describe a safe method for making pure crystals of copper sulfate from copper carbonate
and dilute sulfuric acid. Use the information in the figure above to help you.
In your method you should name all of the apparatus you will use
- sulfuric acid in beaker (or similar)
- add copper carbonate one spatula at a time
- until copper carbonate is in excess or until no more effervescence occurs *
- filter using filter paper and funnel
- filter excess copper carbonate
- pour solution into evaporating basin / dish
- heat using Bunsen burner
- leave to crystallise / leave for water to evaporate / boil off water
- decant solution
- pat dry (using filter paper)
- wear safety spectacles / goggles
The student measured the change in mass of the reactants.
Describe another method, other than measuring the change in mass of the reactions, that
the student could have used to find the rate of the reaction between marble chips and hydrochloric acid
collect the gas in a gas syringe
measured the volume of gas
Give two reasons why quarrying is bad for the environment
any two from: • visual pollution • noise pollution • dust pollution • habitat destruction.
Suggest one reason why it is important for the company to calculate the mass of reactants in Stage 3
any one from:
• so no reactant is wasted / left unreacted
• so they know how much product they will make
• need to record / compensate for the carbon dioxide produced
What is the total number of protons and neutrons in an atom called
The mass number
What is meant by a strong acid
fully ionised
In aqueous solution
