How many molecules are in a spoonful of water? — questions

1. deduce

Supplement. A pupil weighs out 1 mol of sulfur, S, and 1 mol of zinc, Zn. (Ar: S 32, Zn 65.) Which statement about the two samples is correct?

Circle one answer.

• Both samples contain the same number of atoms
• Each sample has a mass of 6.02 × 10²³ grams
• The zinc holds more atoms, as zinc atoms are heavier
• They have the same mass, because each is 1 mol

Nudge (only if you are stuck): Look at the Ar values: does a sulfur atom have the same mass as a zinc atom?

2. calculate

Supplement. Calculate the molar mass of (a) carbon dioxide, CO₂, and (b) calcium carbonate, CaCO₃. (Ar: C 12, O 16, Ca 40.)

(a) Molar mass of CO₂: ______________ unit ________

(b) Molar mass of CaCO₃: ______________ unit ________

Nudge (only if you are stuck): Add up the Ar values as you did for Mr, then give the unit g/mol.

3. calculate

Supplement. Calculate the amount of substance in (a) 14.9 g of potassium chloride, KCl (M = 74.5 g/mol), and (b) 4.25 g of ammonia, NH₃. (Ar: H 1, N 14.) Give each calculated answer to three significant figures.

(a) Amount of potassium chloride: ______________ unit ________

(b) Amount of ammonia: ______________ unit ________

Nudge (only if you are stuck): amount = mass ÷ molar mass. For ammonia, find M first.

4. calculate

Supplement. (a) Calculate the amount of copper(II) oxide, CuO, in 6.00 g. (b) Calculate the mass of 0.350 mol of magnesium oxide, MgO. (Ar: O 16, Mg 24, Cu 64.) Give each calculated answer to three significant figures.

(a) Amount of copper(II) oxide: ______________ unit ________

(b) Mass of magnesium oxide: ______________ unit ________

Nudge (only if you are stuck): Find each molar mass first. Is the unknown an amount or a mass?

5. calculate / deduce / identify

Supplement. 0.120 mol of a metal has a mass of 4.80 g. (a) Calculate its molar mass. Give it to three significant figures. (b) Deduce its relative atomic mass. (c) Identify the metal, using the Periodic Table.

(a) Molar mass: ______________ unit ________

(b) Relative atomic mass: ______________ unit ________

(c) The metal

Nudge (only if you are stuck): Molar mass is the mass of one mole: which quantity goes on top?

6. deduce

Supplement. Each sample is one mole of a substance. Match each sample to the amount of the named particles it holds.

Write the number of the matching option.

1. 1 mol
2. 2 mol
3. 3 mol
4. 4 mol

• Atoms in 1 mol of ammonia, NH₃: ______
• Ions in 1 mol of magnesium chloride, MgCl₂: ______
• Atoms in 1 mol of argon, Ar: ______
• Al³⁺ ions in 1 mol of aluminium oxide, Al₂O₃: ______
• Oxygen atoms in 1 mol of sulfur dioxide, SO₂: ______

Nudge (only if you are stuck): Name the particle: how many of them are in one formula?

7. calculate

Supplement. A sample of nitrogen dioxide, NO₂, is 0.350 mol. Calculate the number of (a) molecules and (b) atoms in the sample. Give each calculated answer to three significant figures. (Avogadro constant: 6.02 × 10²³ per mol.) You can type 1.23 × 10²³ as 1.23x10²³.

(a) Molecules: ______________ unit ________

(b) Atoms: ______________ unit ________

Nudge (only if you are stuck): Count the molecules first. Then ask how many atoms each molecule has.

8. calculate

Supplement. A sample of methane gas, CH₄, contains 2.408 × 10²³ molecules. (a) Calculate the amount of methane. (b) Calculate its mass. Give each calculated answer to three significant figures. (Avogadro constant: 6.02 × 10²³ per mol.) (Ar: H 1, C 12.)

(a) Amount of methane: ______________ unit ________

(b) Mass of methane: ______________ unit ________

Nudge (only if you are stuck): Go along the route map from particles to moles, then from moles to grams.

9. calculate

Supplement. Two pots each hold 5.40 g of a white solid: one holds glucose, C₆H₁₂O₆, and the other urea, CO(NH₂)₂. (a) Calculate the number of molecules in the glucose. (b) Calculate the number of molecules in the urea. (c) Calculate how many times as many molecules the urea pot holds as the glucose pot. Give each calculated answer to three significant figures. (Avogadro constant: 6.02 × 10²³ per mol.) (Ar: H 1, C 12, N 14, O 16.) You can type 1.23 × 10²³ as 1.23x10²³.

(a) Molecules of glucose: ______________ unit ________

(b) Molecules of urea: ______________ unit ________

(c) Times as many: ______________ unit ________

Nudge (only if you are stuck): Same mass, different molar masses: which pot holds more moles?