Key Calculations quiz

1. Complete the following expression: mass =

amount (in moles) x Mᵣ

Aᵣ / Mᵣ

amount (in moles) / Mᵣ

Mᵣ / amount (in moles)

Question 1 of 17

2. In a chemical reaction, the overall molar enthalpy is +87 kJ/mol. Is this reaction exothermic or endothermic?

Depends on the reagents

Endothermic

Exothermic

Depends on the temperature of the surroundings

Question 2 of 17

3. Use Q=mcΔT and c=4.18J/°C/g. A spatula of sodium fluoride is put into a boiling tube with 25cm³ of water. The temperature drops from 22.0°C to 18.6°C. What is the total heat energy change?

1944 kJ

355 kJ/mol

355 J

1944 J

Question 3 of 17

4. What mass of calcium bromide will fully react with 71g of chlorine?

120 g

200g

240 g

480 g

Question 4 of 17

5. To determine the formula of a metal oxide by combustion, magnesium is heated in a crucible. Why is a lid lifted from time to time?

To allow some magnesium oxide smoke to escape

To allow carbon dioxide to escape

To stop the crucible heating up too much

To allow oxygen in so the magnesium fully reacts

Question 5 of 17

6. 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 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 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 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 three concordant results (within 0.2cm³ of each other). 7) Result is the average of all concordant results.

Question 6 of 17

7. State the expression for calculating % yield.

% yield = (theroretical amount of products/actual amount of products) x 100

% yield = (actual amount of products/theoretical amount of products)

% yield = (actual amount of products/theoretical amount of products) x 100

% yield = (theroretical amount of products/actual amount of products)

Question 7 of 17

8. 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 50g water?

0.68g

68g

17g

0.17g

Question 8 of 17

9. What is meant by the term empirical formula?

A method of calculating the ratios of masses in an equation

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

Baby food for emperors

Question 9 of 17

10. What volume does 0.5 moles of ethene gas occupy (at rtp)

12 cm³

12 dm³

48 dm³

48 cm³

Question 10 of 17

11. An oxide of nitrogen contains 26% nitrogen and 74% oxygen and has a relative molecular mass of 108. Find the empirical and molecular formulae for the oxide.

Empirical formula is N₄O₁₀. Molecular formula is N₂O₅

Empirical formula is N₂O₅. Molecular formula is N₄O₁₀

Empirical formula is N₂O₅. Molecular formula is also N₂O₅

Empirical formula is NO₂.₅ Molecular formula is N₂O₅

Question 11 of 17

12. A sample of bromine contained the two isotopes in the following proportions: bromine-79 = 50.7% and bromine-81 = 49.3%. Calculate the relative atomic mass of bromine

((81x50.7)+(79x49.3))/100 = 80.01

(81 + 79)/2 = 80

((79x50.7)+(81x49.3))/100 = 79.99

(81x79)/100 = 63.99

Question 12 of 17

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

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

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

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

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

Question 13 of 17

14. State the expression for calculating molar concentration

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

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

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

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

Question 14 of 17

15. What is the empirical formula of an alcohol with the analysis; 52.2% C, 13.0% H and 34.8% O?

C₂H₆O₂

C₂H₆O

C₂H₃O

CH₆O

Question 15 of 17

16. What are the units for amount in Chemistry?

Grams

g per 100g of solvent

Moles

mol/dm³

Question 16 of 17

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

44

38

22

88

Question 17 of 17

Key Calculations quiz

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