Moles and Calculations

1. Equations must be balanced because atoms are never created or destroyed, which is the law of conservation of ____.

• masscorrect
• charge
• energy
• volume

The law of conservation of mass means the number of each type of atom stays the same.

2. The reactant that is completely used up first in a reaction is called the limiting reagent.

Answer: True

The limiting reagent runs out first and controls the maximum amount of product that can form.

3. In the reaction N2 + 3H2 -> 2NH3, how many moles of hydrogen react with 1 mole of nitrogen?

Answer: 3 moles

The balancing numbers show a 1:3 ratio of nitrogen to hydrogen, so 3 moles of H2 are needed.

4. Which is the correctly balanced equation for hydrogen burning in oxygen to form water?

• 2H2 + O2 -> 2H2Ocorrect
• H2 + O2 -> H2O
• H2 + O2 -> 2H2O
• 2H2 + 2O2 -> 2H2O

2H2 + O2 -> 2H2O has 4 hydrogen atoms and 2 oxygen atoms on each side.

5. Put these steps for a reacting-mass calculation into the correct order.

Answer: Write the balanced equation -> Convert the known mass to moles -> Use the mole ratio to find moles of the unknown -> Convert the moles of the unknown back to a mass

You balance the equation, convert the known mass to moles, use the mole ratio, then convert back to a mass.

6. Which statements about a reactant that is in excess are correct? (Select all that apply.)

• Some of it remains unreacted at the endcorrect
• It is not completely used upcorrect
• It controls the maximum amount of product
• There is more of it than is needed to react with the limiting reagentcorrect

An excess reactant is more than enough, so some remains unreacted at the end; it is the limiting reagent, not the excess one, that controls the amount of product.

7. Concentration in mol/dm3 is found by dividing the number of moles by the ____ in dm3.

• volumecorrect
• mass
• temperature
• pressure

Concentration (mol/dm3) = moles / volume, so a smaller volume of the same amount gives a higher concentration.

8. Arrange the words to state what happens to solute during a dilution.

Answer: diluting a solution keeps the moles of solute constant

Diluting a solution keeps the moles of solute constant; only the volume and concentration change.

9. What is the concentration, in mol/dm3, of a solution containing 0.5 moles dissolved in 2 dm3?

Answer: 0.25 mol/dm3

concentration = moles / volume = 0.5 / 2 = 0.25 mol/dm3.

10. What is the concentration of a solution with 2 moles of solute in 0.5 dm3?

• 4 mol/dm3correct
• 1 mol/dm3
• 2.5 mol/dm3
• 0.25 mol/dm3

concentration = moles / volume = 2 / 0.5 = 4 mol/dm3.

11. Which of these is NOT a unit of concentration?

• cm3correct
• mol/dm3
• g/dm3
• mol/L

cm3 is a unit of volume, whereas mol/dm3, g/dm3 and mol/L all measure concentration.

12. Put these back-titration steps into the correct order.

Answer: Add a measured excess of acid to the solid sample -> Allow the solid to react fully with the acid -> Titrate the leftover acid against a standard alkali -> Find the moles of acid that reacted with the solid -> Calculate the amount of solid originally present

You react the solid with a known excess of acid, titrate the leftover acid, find how much acid reacted with the solid, and finally work out the amount of solid present.

13. The relative formula mass (Mr) of a compound is found by adding up the relative ____ masses of every atom in its formula.

• atomiccorrect
• molecular
• nuclear
• ionic

Mr is the sum of the relative atomic masses (Ar) of all the atoms shown in the chemical formula.

14. What is the relative formula mass (Mr) of carbon dioxide, CO2? (Ar: C = 12, O = 16)

Answer: 44

12 + (2 x 16) = 12 + 32 = 44.

15. Match each compound to its relative formula mass (Mr).

Answer: H2O = 18; CO2 = 44; NaCl = 58.5; CaCO3 = 100

H2O = 18, CO2 = 44, NaCl = 58.5, CaCO3 = 100, found by adding the relative atomic masses.

16. What is the relative formula mass (Mr) of calcium carbonate, CaCO3? (Ar: Ca = 40, C = 12, O = 16)

• 100correct
• 68
• 84
• 116

40 + 12 + (3 x 16) = 40 + 12 + 48 = 100.

17. Relative atomic mass has no units.

Answer: True

Relative atomic mass compares an atom's mass to 1/12 of a carbon-12 atom, so it is a ratio with no units.

18. 0.24 g of magnesium combines exactly with 0.16 g of oxygen. Spell the empirical formula of the oxide formed. (Ar: Mg=24, O=16)

Answer: MgO

Moles Mg = 0.24/24 = 0.01 and moles O = 0.16/16 = 0.01, a 1:1 ratio, so the formula is MgO.

19. What is the approximate value of Avogadro's constant, the number of particles in one mole?

• 6.02 x 10^23correct
• 3.14 x 10^10
• 1.60 x 10^-19
• 9.81 x 10^2

One mole of any substance contains 6.02 x 10^23 particles, known as Avogadro's constant.

20. Arrange the words to complete the rule for finding the number of particles.

Answer: number of particles = moles x Avogadro's constant

Number of particles = number of moles multiplied by Avogadro's constant.

21. Exactly 12 g of carbon-12 contains one mole of carbon atoms.

Answer: True

The mole is defined so that 12 g of carbon-12 contains 6.02 x 10^23 atoms, which is one mole.

22. The number of particles in one mole is given by the ____ constant.

• Avogadro'scorrect
• Planck's
• Faraday's
• Boltzmann's

Avogadro's constant, 6.02 x 10^23 per mole, links the number of moles to the number of particles.

23. How many moles of atoms are present in 1.204 x 10^24 atoms of neon?

Answer: 2 moles

Dividing 1.204 x 10^24 by Avogadro's constant (6.02 x 10^23) gives 2 moles.

24. Match each symbol in the ideal gas equation pV = nRT to what it represents.

Answer: p = Pressure; V = Volume; n = Amount in moles; T = Temperature in kelvin

In pV = nRT: p is pressure, V is volume, n is the amount in moles, and T is the temperature in kelvin.

25. Which formula gives the percentage yield of a reaction?

• (actual yield / theoretical yield) x 100correct
• (theoretical yield / actual yield) x 100
• actual yield x theoretical yield
• (actual yield + theoretical yield) / 2

Percentage yield = (actual yield / theoretical yield) x 100.

26. Atom economy measures the proportion of reactant atoms that end up in the ____ product.

• desiredcorrect
• waste
• gaseous
• solid

A high atom economy means most reactant atoms become the useful (desired) product rather than waste.

27. A reaction has a theoretical yield of 10 g but only 8 g of product is actually made. What is the percentage yield?

Answer: 80 %

(8 / 10) x 100 = 80%.

28. A percentage yield can never be greater than 100%.

Answer: True

You cannot make more product than the theoretical maximum, so yield is at most 100%.

29. Which of the following can cause the actual yield of a reaction to be less than the theoretical yield? Select all that apply.

• The reactants do not fully reactcorrect
• Some product is lost during filtering or transfercorrect
• Unwanted side reactions make different productscorrect
• Atoms are destroyed during the reaction

Incomplete reactions, product lost during separation, and unwanted side reactions all lower yield; atoms are never actually destroyed.

30. Sort each idea by whether it tells you about how efficiently atoms are used or how much product is actually made.

Answer: Atom economy = Atom efficiency; Waste made per unit of useful product = Atom efficiency; Percentage yield = Amount of product made; Actual mass divided by theoretical mass = Amount of product made

Atom economy and waste per useful product describe atom efficiency, while percentage yield and actual-vs-theoretical mass describe how much product is made.