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 reaction has an activation energy of \(40kJmol^{−1}\) and an enthalpy change of \(-60kJ mol^{-1}\)
Which potential energy diagram illustrates this reaction?
Answer/Explanation
Ans: C
Question
Which of the $0.001 \mathrm{~mol} \mathrm{dm}^{-3}$ solutions is most likely to have a pH of 11.3 ?
A. $\mathrm{Ca}(\mathrm{OH})_2(\mathrm{aq})$
B. $\mathrm{H}_3 \mathrm{PO}_4(\mathrm{aq})$
C. $\mathrm{NaOH}(\mathrm{aq})$
D. $\mathrm{NH}_4 \mathrm{OH}(\mathrm{aq})$
Answer/Explanation
Solution:
The most likely solution to have a pH of 11.3 would be the one with a base. Among the given options, $\mathrm{Ca}(\mathrm{OH})_2(\mathrm{aq})$ is a strong base, which will readily dissociate in water to give $\mathrm{OH}^{-}$ ions. Therefore, it is the most likely solution to have a pH of 11.3.
The dissociation reaction for $\mathrm{Ca}(\mathrm{OH})_2(\mathrm{aq})$ is:
$\mathrm{Ca}(\mathrm{OH})_2(\mathrm{aq}) \rightarrow \mathrm{Ca}^{2+}(\mathrm{aq})+2 \mathrm{OH}^{-}(\mathrm{aq})$
Since each mole of $\mathrm{Ca}(\mathrm{OH})_2(\mathrm{aq})$ gives two moles of $\mathrm{OH}^{-}$ ions, the concentration of $\mathrm{OH}^{-}$ ions in the solution will be:
$\left[\mathrm{OH}^{-}\right]=2 \times 0.001=0.002 \mathrm{~mol} \mathrm{dm}^{-3}$
Using the relationship $pH + pOH = 14$, we can calculate the pH of the solution:
$\begin{aligned} & p O H=-\log \left(\left[\mathrm{OH}^{-}\right]\right)=-\log (0.002) \approx 2.70 \\ & p H=14-p O H=14-2.70 \approx 11.30\end{aligned}$
${\colorbox{yellow}{Correct Option : A. $\mathrm{Ca}(\mathrm{OH})_2(\mathrm{aq})$.}$
Question
Which species could be reduced to form $\mathrm{NO}_2$ ?
A. $\mathrm{N}_2$
B. $\mathrm{NO}_3^{-}$
C. $\mathrm{HNO}_2$
D. $\mathrm{NO}$
Answer/Explanation
Solution:
In order to form $\mathrm{NO}_2$ through reduction, we need a species that contains nitrogen and oxygen, and has a higher oxidation state than $\mathrm{NO}$ which has an oxidation state of +2. Among the options given, only $\mathrm{NO}_3^{-}$ has nitrogen and oxygen and a higher oxidation state than $\mathrm{NO}$. Therefore, the $\colorbox{yellow}{Correct Option: B. $\mathrm{NO}_3^{-}$.}$