C4 - Chemical Calculations Flashcards
What is the unit for Volume?
cm³ or dm³
What is unit for mass?
g or kg
What is the unit for amount?
Mol
What is the apparatus used to measure Volume?
Measuring cylinder / burette / volumetric flask
What is the apparatus used to measure mass?
mass balance
What is Avogadro’s constant?
6.02x10²³
What does Avogadro’s constant represent?
the number of particles in 1 mole of particles
What is an example of a chemical formula?
H₂O
What is a burette?
a long glass tube with a tap at one end and markings to show volumes of liquid; used to add precisely known volumes of liquids to a solution in a conical flask below it
What is concentration?
the amount of a substance dissolved in a given volume of liquid
What are concordant results?
when you have two titration results that are within 0.1cm³ of each other. These precise results can then be used to calculate an accurate mean
What is the end point?
the point in a titration where the reaction is complete and titration should stop
What is a limiting reactant?
the reactant in a chemical reaction that when used up causes the reaction to stop
What is percentage yield?
the actual mass of product collected in a reaction divided by the maximum mass that could have been formed in theory, multiplied by 100
What is a pipette?
a glass tube used to measure accurate volumes of liquids
What is relative atomic mass, Aᵣ?
the average mass of the atoms of an element compared with carbon-12 (which is given a mass of exactly 12). The average mass must take into account the proportions of the naturally occurring isotopes of the element
What is Relative formula mass, Mr?
the total of the relative atomic masses, added up in the ratio shown in the chemical formula, of a substance
What is titration?
a method for measuring the volumes of two solutions that react together
What is yield?
the mass of product that a chemical reaction produces
What is an example of a balanced chemical equation?
2H₂ + O₂ -> 2H₂O
What does a chemical formula show?
The mole ratio of atoms within a compound using subscripts after each atom
What does an empirical formula show?
The simplest (whole number) mole ratio of atoms of each element present within a compound
What does a molecular formula show?
The actual (whole number) mole ratio of atoms of each element present within a molecule
What is the formula for the number of moles?
Number of moles = Mass (g) / Molar Mass
n = m/M
What is the formula for molar mass?
Molar mass = Mass (g) / Number of moles
M = m/n
What is the law of conservation of mass?
Total mass of reactants = Total mass of products
What is the limiting reactant in the equation “Ca + H₂O -> Ca(OH)₂” when given 2 mol of Ca and 10 mol of H₂O?
there are 8 moles of H₂O in excess so Ca is the limiting reactant
How much Ca(OH)₂ is produced in the equation “Ca + H₂O -> Ca(OH)₂” when given 2 mol of Ca and 10 mol of H₂O?
2 Moles
What is the limiting reactant in the equation “2Mg + O₂ -> 2MgO” when given 10 mol of Mg and 3 Mol of O₂?
There are 4 moles of Mg in excess so O₂ is the limiting reactant
How much MgO is produced, in moles, in the equation “2Mg + O₂ -> 2MgO” when given 10 mol of Mg and 3 Mol of O₂?
6 Moles
What mass of aluminium can be formed from 1020g of aluminium oxide in the formula “2Al₂O₃ -> 4Al + 3O₂
n(2Al₂O₃) = 1020/102 = 10 mol
n(4Al) = 2x10 = 20 mol m(4Al) = 27x20 = 540g
What mass of NH₃ is needed to make 148g of N₂H₄ in the formula “2NH₃ + H₂O₂ -> N₂H₄ + 2H₂O”?
n(N₂H₄) = 148/32 = 4.625 mol
n(2NH₃) = 2 x 4.625 = 9.25 mol m(2NH₃) = 9.25 x 17 = 157.25g
10g of hydrated sodium sulfate decompose to form 4.4g of anhydrous sodium sulfate on heating. Calculate the value of x in the formula “Na₂SO₄.xH₂O -> Na₂SO₄ + xH₂O”
n(Na₂SO₄) = 4.4/142 = 0.03 mol
n(xH₂O) = 0.03 X x = 0.03x mol
m(xH₂O) = 0.03x X 18 = 5.6
0.55x = 5.6
x = 10.04
Calculate the mass of oxygen needed to react 9.6g of magnesium to form magnesium oxide in the equation “2Mg + O₂ -> 2MgO”
n(2Mg) = 9.6/24 = 0.4 mol
n(O₂) = 0.4/2 = 0.2 Mol m(O₂) = 0.2 x 32 = 6.4g
What reasons might cause a reaction to not give a 100% yield?
- Reversible Reaction
- Side reactions
- Losses in separation
- Losses in purification
- Some reactants may be left in apparatus
What is the equation for percentage yield?
% yield = (actual mass of product formed / maximum theoretical mass of products formed) x 100
Iron is made by the reduction of iron oxide with carbon monoxide. Calculate the mass of iron that can be formed from 126g of iron oxide, then calculate the percentage yield if 78.5g of iron is formed in the equation “Fe₂O₃ + 3CO -> 2Fe + 3CO₂”
n(Fe₂O₃) = 126/160 = 0.7875 mol
n(2Fe) = 2 x 0.7875 = 1.575 mol m(2Fe) = 1.575 x 56 = 88.2g
% yield = (78.5/88.2) x 100 = 89%
What do chemical companies need to consider when using reactions to make products they sell?
- Percentage Yield
- Reaction Rate
- Reversibility
- Energy Cost
- Cost of raw materials/reactants
- Environmental impact
- Waste Products
What is atom economy?
the % of the mass of the reactants that become useful products
What is the equation for atom economy?
Atom Economy = (Mr of desired product from equation / sum of Mr of all reactants from equation) x 100
Why is it important to maximise Atom Economy in industrial processes?
- It saves money
- It conserves earths resources
- It minimises pollution
What is the atom economy of the equation “CH₄ + H₂O -> CO + 3H₂” if Hydrogen is the desired product?
Mr of reactants = 16 + 18 = 34
Mr of desired product = 6
Atom Economy = (6/34) x 100 = 17.6%
What is the atom economy of the equation “WO₃ + 3H₂ -> W + 3H₂O “ if W is the desired product?
Mr of reactants = 238
Mr of desired product = 184
Atom Economy = (184/238) x 100 = 77%
What is the atom economy of the equation “CuCO₃ -> CuO + CO₂” if CuO is the desired product?
Mr of reactants = 123.5
Mr of desired product = 79.5
Atom Economy =(79.5/123.5) x 100 = 64%
6.15g of hydrated magnesium sulfate decompose to form 3g of anhydrous magnesium sulfate on heating in the equation “MgSO₄.xH₂O -> MgSO₄ + xH₂O” Calculate the value of x
n(MgSO₄) = 3/120 = 0.025 mol
n(xH₂O) = x X 0.025 = 0.025x mol
m(xH₂O) = 0.025x X 18 = 3.15g
0.025x = 0.175
x = 7
Calculate the amount, in moles, of CaO formed if 10 mol of calcium reacts with 2 mol of oxygen in the equation “2Ca + O₂ -> 2Cao”
4 mol
Equal amounts, in moles, of any gas occupy the same volume under what consditions?
- The same temperature
* The same pressure
What is the equation for the amount of gas in moles?
Amount of gas (mol) = Volume (dm³) / 24
The volume of 1 mole of any gas at room temperature and room pressure is …….
24dm³
What is room temperature?
20°C
What is room pressure?
1 atm
Find the volume of hydrogen gas (at room temperature and pressure) formed when 0.5g of calcium reacts with hydrochloric acid in the equation “Ca(s) + 2HCl(aq) -> CaCl₂(aq) + H₂”
n(Ca) = 0.5/40 = 0.0125
n(H₂) = 0.0125 x 1 = 0.0125 v(H₂) = 24 x 0.0125 = 0.3
What is Molar Mass (M)?
the mass per mole of a substance
What is the unit for molar mass?
g/mol
What is the equation for mass?
mass (g) = amount (mol) x molar mass (g/mol)
What is the equation for volume in a gas?
Volume = 24 x amount of gas (mol)
Calculate the minimum mass of sodium hydrogen carbonate that must be heated to produce 0.25dm³ or CO₂ in the equation “2NaHCO₃ -> Na₂CO₃ + H₂O + CO₂”
n(CO₂) = 0.25/24 = 1/96 mol
n(2NaHCO₃) = 1/96 x 2 = 1/48 mol m(2NaHCO₃) = 1/48 x 84 = 1.75g
Calculate the number of moles present in each following volumes:
a) 96dm³ of Helium
b) 0.06dm³ of Argon
c) 48000cm³ of hydrogen
d) 1920cm³ of chlorine
a) 96/24 = 4 mol
b) 0.06/24 = 1/400 mol
c) 48/24 = 2 mol
d) 1.92/24 = 0.08 mol
What is 1000cm³ in dm³?
1dm³
What is 1dm³ in cm³?
1000cm³
Calculate the volume of the following gases:
a) 4 Mol of Oxygen
b) 0.25 Mol of Methane
c) 15g of Argon
d) 0.2g of CO₂
a) 24 x 4 = 96 dm³
b) 0.25 x 24 = 6 dm³
c) (15/40) x 24 = 9 dm³
d) (0.22/44) x 24 = 0.12 dm³
What is the 3-step method to calculate reacting masses, gases and solutions?
1) Convert the data given in the question into amount, in moles
2) Use the mole ratio of the substances in the balanced chemical equation to calculate the amount, in moles, of the substance required
3) Concert the amount, in moles, back into the data required i.e. mass (g) or volume (dm³) or concentration (mol/dm³ or g/dm³)
Calculate the mass of the following gases:
a) 7.2 dm³ of ammonia
b) 480 cm³ of Nitrogen
c) 100cm³ of oxygen
a) (7.2/24) x 17 = 5.1g
b) (0.48/24) x 28 = 0.56g
c) (0.1/24) x 32 = 0.13g
Find the volume of carbon dioxide gas formed when 6kg of ethane burns in oxygen in the formula “2C₂H₆(g) + 7O₂(g) -> 4CO₂(g) + 6H₂O(l)”
n(2C₂H₆) = 6000/30 = 200 mol
n(4CO₂) = 200 x 2 = 400 mol v(4CO₂) = 400 x 24 = 9600 dm³
Find the volume of oxygen needed to complete combustion with 79.2g of propane in the equation “C₃H₈(g) + 5O₂(g) -> 3CO₂(g) + 4H₂O(g)”
n(C₃H₈) = 79.2/44 = 1.8 mol
n(5O₂) = 5 x 1.8 = 9 mol v(5O₂) = 9 x 24 = 216 dm³
What is concentration?
The concentration of a solution is the amount of substance per unit volume
What are the units for concentration?
mol/dm³
g/dm³
What are the equations for concentration?
concentration (mol/dm³) = amount (mol) / volume (dm³)
concentration (g/dm³) = mass (g) / volume (dm³)
What is the concentration of the following solutions in mol/dm³?
a) 0.1 moles of NaCl in 200 cm³
b) 0.2 moles of H₂SO₄ in 100 cm³
c) 0.02 moles of NaOH in 25 cm³
a) 0.1/0.2 = 0.5 mol/dm³
b) 0.2/0.1 =2 mol/dm³
c) 0.02/0.025 = 0.8 mol/dm³
Calculate the number of moles in the following substances
a) 100 cm³ of 0.2 mol/dm³ HNO₃
b) 25 cm³ of 1.5 mol/dm³ KOH
c) 50 cm³ of 0.1 mol/dm³ H₂SO₄
a) 0.2 x 0.1 = 0.02 mol
b) 1.5 x 0.025 = 0.0375 mol
c) 0.1 x 0.05 = 0.005 mol
Calculate the concentration of the following substances in g/dm³
a) 0.1 mol/dm³ NaOH
b) 0.25 mol/dm³ CH₃COOH
c) 1.5 mol/dm³ HNO₃
a) 40 x 0.1 = 4 g/dm³
b) 60 x 0.25 = 15 g/dm³
c) 63 x 1.5 = 94.5 g/dm³
Calculate the concentration in g/dm³ when 0.2 moles of NaOH is dissolved in 250 cm³ of water
m(NaOH) = 0.2 x 40 = 8g c(NaOH) = 8/0.25 = 32g/dm³
What is temperature measured in?
°C
What is used to measure temperature?
thermometer
What is uncertainty?
Whenever a measurement is made there is always uncertainty about the result obtained
What is uncertainty recorded as?
It is recorded as a ± value (e.g 21 ± 2°C)
How do you calculate the mean value and uncertainty?
Mean value of data ± half the range
What is the mean value and uncertainty of the results: 3.8, 4.0, 4.1, 3.9, 4.2?
4±0.2
What is the mean value and uncertainty of the results: 24.2, 25.6, 24.4, 24.1?
24.23±0.15
What is the mean value and uncertainty of the results: 32, 28, 26, 46, 34, 21, 30
30±4
25cm³ of a solution of sodium hydroxide solution required 21.5cm³ of 0.1 mol/dm³ sulfuric acid for neutralisation. Find the concentration of sodium hydroxide solution in mol/dm³ given the equation “H₂SO₄(aq) + 2NaOH(aq) -> Na₂SO₄(aq) + 2H₄O(l)”
n(H₂SO₄) = 0.1 x 0.0215 = 0.00215 mol
n(2NaOH) = 2 x 0.00215 = 0.0043 mol c(2NaOH) = 0.0043/0.025 = 0.172 mol/dm³
Find the volume of 1.2 mol/dm³ hydrochloric acid that reacts with 25 cm³ of 1.5 mol/dm³ sodium hydroxide in the equation “HCl (aq) + NaOH(aq) -> NaCl(aq) + H₂O(l)”
n(NaOH) = 1.5 x 0.025 = 0.0375 mol
n(HCl) = 1 x 0.0375 = 0.0375 mol v(HCl) = 0.0375/1.2 = 1/32dm³
25 cm³ of arsenic acid, H₃AsO₄ , required 37.5 cm³ of 0.1 mol/dm³ sodium hydroxide for neutralisation in the equation “3NaOH(aq) + H₃AsO₄(aq) -> Na₃AsO₄(aq) + 3H₂O2”. Find the concentration of the arsenic acid in mol/dm³
n(3NaOH) = 0.1 x 0.0375 = 0.00375 mol
n(H₃AsO₄) = 0.00375/3 = 0.00125 mol c(H₃AsO₄) = 0.00125/0.025 = 0.05 mol/dm³
What volume of 0.04 mol/dm³ calcium hydroxide just neutralises 25 cm³ of 0.1 mol/dm³ nitric acid in the equation “Ca(OH)₂(aq) +2HNO₃(aq) -> Ca(NO₃)₂(aq) + 2H₂O(l)”
n(2HNO₃) = 0.1 x 0.025 = 0.0025 mol
n(Ca(OH)₂) = 0.0025/2 = 0.00125 mol v(Ca(OH)₂) = 0.00125/0.04 =0.03125 dm³
Describe the method to carry out a titration
1) Collect some hydrochloric acid in a beaker
2) Fill a burette with the hydrochloric acid beyond the zero mark and then let the solution run out until the bottom of the meniscus is exactly on the zero mark. All bubbles should be removed from the jet.
4) Collect some sodium hydroxide in another beaker
6) Use the pipette and the pipette filler to transfer 25.0 cm3 of the sodium hydroxide into a clean dry conical flask.
7) Add three to four drops of phenolphthalein indicator into the flask and swirl. Place the conical flask on the white tile directly below the burette.
8) Record the initial burette reading in the table below (this should be 0.00 cm3).
9) Carry out a rough titration by adding the acid to the alkali in small amounts at a time. Swirl the flask after every addition and continue until the indicator changes from pink to colourless. Note the final burette reading and record it in the table. Your teacher will show you how to read the burette.
10) Repeat the titration accurately by adding the acid drop-wise near the end point. Make sure you record the initial and final burette readings in the appropriate column in the table.
11) Repeat the accurate titrations until you have two concordant results (within 0.10 cm3 of each other).
What colour is phenolphthalein in an acid?
colourless
What colour is phenolphthalein in an alkali?
pink
What colour is methyl orange in an acid?
red
What colour is methyl orange in an alkali?
yellow
22.5 cm³ of sodium hydroxide solution reacted with 25 cm³ of 0.1 mol/dm³ hydrochloric acid in the equation “NaOH(aq) + HCl(aq) -> NaCl(aq) + H₂O(l)” Calculate the concentration of the sodium hydroxide solution in mol/dm³
n(HCl) = 0.1 x 0.025 = 0.0025 mol
n(NaOH) = 1 x 0.0025 = 0.0025mol c(NaOH) = 0.0025/0.0225 = 0.11 mol/dm³
Magnesium burns in oxygen gas to produce magnesium oxide. When 9.6g of Magnesium is heated, 16g of magnesium oxide is formed. Determine the balanced chemical equation for the reaction of magnesium with oxygen.
m(O₂) = 16 - 9.6 = 6.4g
n(Mg) = 9.6/24 = 0.4 mol n(O₂) = 6.4/32 = 0.2 mol n(MgO) = 16/40 = 0.4 mol
0.4Mg + 0.2O₂ -> 0.4MgO = 2Mg + O₂ = 2MgO
Sodium nitrate, NaNO₃, decomposes on heating to give sodium nitrite, NaNO₂, and oxygen gas, O₂. When 8.5g of sodium nitrate is heated, 6.9g of sodium nitrite is produced. Determine a balanced chemical equation for the decomposition of sodium nitrate.
m(O₂) = 8.5 - 6.9 = 1.6
n(NaNO₃) = 8.5/85 = 0.1 n(NaNO₂) = 6.9/69 = 0.1 n(O₂) = 1.6/32 = 0.05
0.1NaNO₃ -> 0.1NaNO₂ + 0.05O₂ = 2NaNO₃ -> 2NaNO₂ +O₂
How can you increase the concentration of an aqueous solution?
- Add more solute and dissolve it in the same volume of its solution
- Evaporating off some of the water from the solution so you have the same mass of solute in a smaller volume of solution
What is the reactant that gets used up first in a reaction called?
The limiting reactant
What does sulfur dioxide cause?
acid rain
How do you measure the exact volumes of acid and alkali needed to react together?
Titration
What volume of oxygen gas is needed to react with 4 dm^3 of propane in the reaction: C3H8 + 5O2 → 3CO2 + 4H2O
5x4 = 20
