Key Calculations quiz

1. What are the units for amount in Chemistry?

mol/dm³

g per 100g of solvent

Grams

Moles

Question 1 of 17

2. What is meant by the term empirical formula?

Baby food for emperors

A chemical formula that shows the actual numbers of the different types of atoms in a molecule

A chemical formula that shows the simplest ratio of the numbers of atoms in a compound

A method of calculating the ratios of masses in an equation

Question 2 of 17

3. In a combustion calorimetry experiment, 0.78g of ethanol (C₂H₅OH) produced 12,540 J of heat energy. Calculate the molar enthalpy change.

-213 kJ/mol (Amount = 46/0.78 = 59.0 mol. Answer = 12540/59.0 = 213 kJ/mol)

-369 kJ/mol (Amount = 0.78/23 = 0.034 mol. Answer = 12540/1000/0.034 = 369 kJ/mol)

-738 kJ/mol (Amount = 0.78/46 = 0.017 mol. Answer = 12540/1000/0.017 = 738 kJ/mol)

-425 kJ/mol (Amount = 23/0.78 = 29.5 mol. Answer = 12540/29.5 = 425 kJ/mol)

Question 3 of 17

4. State the steps for calculating empirical formula

1) Aᵣ/mass. 2) divide by the largest. 3) Give the ratio has a whole number. 4) State final empirial formula

1) Aᵣ/mass. 2) divide by the smallest. 3) Give the ratio has a whole number. 4) State final empirial formula

1) mass/Aᵣ. 2) divide by the smallest. 3) Give the ratio has a whole number. 4) State final empirial formula

1) mass/Aᵣ. 2) divide by the largest. 3) Give the ratio has a whole number. 4) State final empirial formula

Question 4 of 17

5. A sample of carbon contained 98.90% carbon-12 and 1.10% carbon-13. Calculate the relative atomic mass of carbon

((12x98.90)+(13x1.10))/100 = 12.01

((13x98.90)+(12x1.10))/100 = 12.99

(12+13)/2 = 12.5

(12x13)/100 = 1.56

Question 5 of 17

6. A graph shows the solubility of sodium chloride in water at 50°C is 34g/100g. At that temperature, what mass of sodium chloride will dissolve in 200g water?

0.34g

17g

0.68g

68g

Question 6 of 17

7. Use Q=mcΔT and c=4.18J/°C/g. 25cm³ of sulfuric acid is put into a boiling tube. The starting temperature is 21°C. A spatula of iron filings is added. After a while the temperature reaches 33°C. What is the total heat energy change?

2.153 kJ

3383 J

1254 J

2153 J

Question 7 of 17

8. Describe how to carry out an acid-alkali titration

1) Pipette 25cm³ of alkali into a conical flask. 2) Add indicator. 3) Fill a burette with acid, record the initial volume. 4) Whilst swirling the flask, add the acid dropwise until the indicator changes colour. 5) Record the volume and calculate the volume of acid which was added. 6) Repeat until two concordant results (within 0.2cm³ of each other). 7) Result is the average of all concordant results.

1) Pipette 25cm³ of alkali into a conical flask. 2) Add indicator. 3) Fill a burette with acid, record the initial volume. 4) Whilst swirling the flask, add the acid dropwise until the indicator is just about to change colour. 5) Record the volume and calculate the volume of acid which was added. 6) Repeat until two concordant results (within 0.2cm³ of each other). 7) Result is the average of all concordant results.

1) Pipette 25cm³ of alkali into a conical flask. 2) Add indicator. 3) Fill a burette with acid, record the initial volume. 4) Whilst swirling the flask, add the acid dropwise until the indicator changes colour. 5) Record the volume and calculate the volume of acid which was added. 6) Repeat until three concordant results (within 0.2cm³ of each other). 7) Result is the average of all concordant results.

1) Pipette 25cm³ of alkali into a conical flask. 2) Add indicator. 3) Fill a burette with acid, record the initial volume. 4) Whilst swirling the flask, add the acid dropwise until the indicator is just about to change colour. 5) Record the volume and calculate the volume of acid which was added. 6) Repeat until three concordant results (within 0.2cm³ of each other). 7) Result is the average of all concordant results.

Question 8 of 17

9. Complete the following expression: Mᵣ =

mass² / amount (in moles)

mass / amount (in moles)

amount (in moles) /mass

mass x amount (in moles)

Question 9 of 17

10. What volume does 3 moles of carbon dioxide gas occupy (at rtp)?

72 cm³

8 cm³

8 dm³

72 dm³

Question 10 of 17

11. In the thermal decomposition of calcium carbonate, we might expect 50g of calcium carbonate to produce 28g of calcium oxide. If instead only 24g of calcium oxide is produced, what is the percentage yield?

0.56 %

85.7 %

56 %

0.857 %

Question 11 of 17

12. A compound that contained 24.24% Carbon. 4.04% Hydrogen and 71.72% Chlorine and has a relative molecular mass of 99. Calculate the empirical formula and the molecular formula.

Empirical formula is C₂H₄Cl₂. Molecular formula is CH₂Cl

Empirical formula is CH₂Cl. Molecular formula is CH₂Cl

Empirical formula is CH₂Cl. Molecular formula is C₂H₄Cl₂

Empirical formula is C₂H₄Cl₂. Molecular formula is C₂H₄Cl₂

Question 12 of 17

13. State the expression for calculating molar concentration

Molar concentration = volume (in dm³) / Amount (in moles)

Molar concentration = Amount (in moles) x volume (in cm³)

Molar concentration = Amount (in moles)/volume (in cm³)

Molar concentration = Amount (in moles)/volume (in dm³)

Question 13 of 17

14. What mass of carbon dioxide is produced when 24g of carbon undergoes completely combustion?

176 g

44 g

24 g

88g

Question 14 of 17

15. Calculate the relative formula mass(Mr) of carbon dioxide (CO₂)

38

22

88

44

Question 15 of 17

16. In a chemical reaction, 250 kJ/mol is taken to break all the bonds and 280 kJ/mol is released when new bonds are formed. What is the overall molar enthalpy of the reaction?

+530 kJ/mol

-530 kJ/mol

+30 kJ/mol

-30 kJ/mol

Question 16 of 17

17. To determine the formula of a metal oxide by combustion, magnesium is heated in a crucible. Why is a lid used?

To stop carbon dioxide escaping

To stop the escape of magnesium

To keep the magnesium warm

To stop the escape of magnesium oxide smoke

Question 17 of 17

Key Calculations quiz

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