Question
What are the units of molar mass?
A. amu
B. g
C. mol \(g^{-1}\)
D. g \(mol^{-1}\)
▶️Answer/Explanation
Markscheme: D
Molar mass is the mass of one mole of a substance and is expressed in grams per mole (g/mol). Therefore, the correct units for molar mass are represented as g/mol. This unit indicates the amount of mass in grams contained in one mole of a given substance.
In the International System of Units (SI), the mole is the base unit for amount of substance, and grams are the base unit for mass. Combining these two units gives the molar mass in grams per mole.
Question
Which information does the molecular formula provide?
A. The simplest ratio of atoms in a molecule
B. The actual numbers of atoms in a molecule
C. The number of molecules in one mole
D. The types of bonds in a molecule
▶️Answer/Explanation
Markscheme : B
A molecule is comprised of two or more atoms that have been chemically combined. A molecular formula is a chemical formula of a molecular compound that shows the kinds and numbers of atoms present in a molecule of the compound.
Question
A student heated a known mass of zinc powder in an open crucible until there was no further mass change and recorded the final mass.
What would the student be able to derive from this data?
I. Percentage composition of zinc oxide
II. Empirical formula of zinc oxide
III. Molecular formula of zinc oxide
A. I and II only
B. I and III only
C. II and III only
D. I, II and III
▶️Answer/Explanation
Markscheme: A
The student would be able to derive the following information:
I. Percentage composition of zinc oxide – Yes, by comparing the initial and final masses, the student can determine the percentage of zinc in the zinc oxide formed.
II. Empirical formula of zinc oxide – Yes, by determining the moles of zinc and oxygen involved in the reaction, the student can find the empirical formula.
III. Molecular formula of zinc oxide – No, the molecular formula cannot be determined from this experiment alone. To find the molecular formula, additional information about the molar mass of the compound is needed.
Therefore, the correct answer is A. I and II only.
Question
What is the total number of atoms in 0.100 mol of \({\text{[Pt(N}}{{\text{H}}_3}{{\text{)}}_2}{\text{C}}{{\text{l}}_2}{\text{]}}\)?
A. 11
B. \(6.02 \times {10^{22}}\)
C. \(3.01 \times {10^{23}}\)
D. \(6.62 \times {10^{23}}\)
Answer/Explanation
Markscheme
D
Examiners report
Question
Which statements are correct about Avogadro’s constant?
I. It is the number of ions in 12 g of sodium hydride, NaH.
II. It is the number of molecules in \({\text{22.4 d}}{{\text{m}}^{\text{3}}}\) of hydrogen gas at 0 °C and 1 atm.
III. It is the number of atoms in 12 g of \(^{{\text{12}}}{\text{C}}\).
A. I and II only
B. I and III only
C. II and III only
D. I, II and III
Answer/Explanation
Markscheme
D
Examiners report
One respondent stated in the G2 form that technically 1 atm is not standard pressure.
In Table 2 of the Data Booklet the conversion “\({\text{1 atm}} = 1.01 \times {10^5}{\text{ Pa}}\)” is given, so students should be familiar that 1 atm or \(1.01 \times {10^5}{\text{ Pa}}\) can be used for standard pressure.
This question proved to be the most difficult in the paper with only 9.95% of the candidates opting for the correct response D, the vast majority, 75.79%, chose response C thinking that choice I is incorrect. Candidates should know that the sodium ion is \({\text{N}}{{\text{a}}^ + }\) (assessment statement 4.1.3) so they should be able to deduce the oxidation number of hydrogen to be \( – 1\) (assessment statement 9.1.2) and that \({{\text{H}}^ – }\) is the negative ion in sodium hydride, NaH. Although hydrogen is usually assigned the oxidation number of \( + 1\), in metallic hydrides the oxidation number of hydrogen is \( – 1\).