CAIE A-Level Chemistry 2.3 Formulas Question Bank

Practise writing formulae and equations, then using composition and mass data to deduce empirical, molecular and hydrated formulae.

Exam points

write balanced equations, ionic equations and state symbols from reaction information
calculate empirical, molecular or hydrated formulae from percentages, masses or Mr data
interpret formula definitions, ion charges and oxidation numbers in short-answer items

Covered syllabus points

2.3.1—Ionic formulas and oxidation numbers 2.3.2—Balanced and ionic equations 2.3.3—Terms: empirical and molecular formula 2.3.4—Terms: anhydrous, hydrated and water 2.3.5—Empirical and molecular formulas

[Maximum number: 5]

Species such as $\mathrm{NH}_{4}{ }^{+}, \mathrm{CO}_{3}{ }^{2-}$ and $\mathrm{PO}_{4}{ }^{3-}$ are examples of molecular ions.

(a)

$\mathrm{NH}_{4}{ }^{+}$ is a Brønsted-Lowry acid.

[ 1 ]

(i)

When $\mathrm{NH}_{4}{ }^{+}(\mathrm{aq})$ is heated with NaOH(aq), a pungent gas is produced.

Write an ionic equation for this reaction.

[ 1 ]

ionic equation $\quad \mathrm{NH}_{4}{ }^{+}+\mathrm{OH}^{-} \rightarrow \mathrm{NH}_{3}+\mathrm{H}_{2} \mathrm{O}$

(b)

There are many naturally occurring hydrated compounds that contain the anion $\mathrm{PO}_{4}{ }^{3-}$ .

[ 4 ]

(i)

Name the anion $\mathrm{PO}_{4}{ }^{3-}$ .

[ 1 ]

(ii)

Struvite is a soft hydrated mineral with $M_{\mathrm{r}}=245.3$ . The anhydrous form of the mineral has the formula $\mathrm{NH}_{4} \mathrm{MgPO}_{4}$ .

Calculate the number of molecules of water of crystallisation in struvite.
Give your answer to the nearest integer. Show your working.

number of molecules of water of crystallisation =

[ 3 ]

(molecular mass of $\mathrm{NH}_{4} \mathrm{MgPO}_{4}$ ) = 137.3

$\left(245.3-M_{r}\left(\mathrm{NH}_{4} \mathrm{MgPO}_{4}\right) \div 18.0\right)$ =6

[Maximum number: 2]

Sucrose is a white crystalline solid, $\mathrm{C}_{12} \mathrm{H}_{22} \mathrm{O}_{11}$ . In reaction Z, sucrose reacts with water in the presence of a catalyst, aqueous hydrochloric acid, to form glucose and fructose.

reaction Z

(a)

Both fructose and glucose contain chiral centres.

[ 1 ]

(i)

Determine the empirical formula of fructose.

[ 1 ]

empirical formula of fructose $\mathrm{CH}_{2} \mathrm{O}$

(b)

Write the equation for the complete combustion of sucrose.

The enthalpy change of reaction $\mathbf{Z}, \Delta H_{r}$ , can be calculated using the enthalpy change of combustion data given in the table.

Table for Question (b) — CAIE A-Level Chemistry AS

[ 1 ]

$\mathrm{C}_{12} \mathrm{H}_{22} \mathrm{O}_{11}+12 \mathrm{O}_{2} \rightarrow 12 \mathrm{CO}_{2}+11 \mathrm{H}_{2} \mathrm{O}$

[Maximum number: 3]

Ethanedioic acid, $\mathrm{HO}_{2} \mathrm{CCO}_{2} \mathrm{H}$ , has a relative molecular mass of 90.0.

(a)

State the empirical formula of ethanedioic acid.

[ 1 ]

$\mathrm{CO}_{2} \mathrm{H}$

(b)

Solid ethanedioic acid reacts with aqueous calcium ions to make a precipitate of calcium ethanedioate, $\mathrm{CaC}_{2} \mathrm{O}_{4}$ .
$\mathrm{CaC}_{2} \mathrm{O}_{4}$ breaks down when heated to form calcium oxide, carbon dioxide and carbon monoxide.

[ 2 ]

(i)

Construct an equation to represent the reaction of $\mathrm{CaC}_{2} \mathrm{O}_{4}$ when heated. Include state symbols.

[ 2 ]

Correct equation:
$\mathrm{CaC}_2\mathrm{O}_4(\mathrm{s}) \rightarrow \mathrm{CaO}(\mathrm{s}) + \mathrm{CO}_2(\mathrm{g}) + \mathrm{CO}(\mathrm{g})$

Marking points:
M1 correct formulae.
M2 balanced equation and state symbols.

\mathrm{CaC}_2\mathrm{O}_4(\mathrm{s}) \rightarrow \mathrm{CaO}(\mathrm{s}) + \mathrm{CO}_2(\mathrm{g}) + \mathrm{CO}(\mathrm{g})

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Covered syllabus points

2.3 Formulas question 1

Question (a)

Question (i)

Question (b)

Question (i)

Question (ii)

2.3 Formulas question 2

Question (a)

Question (i)

Question (b)

2.3 Formulas question 3

Question (a)

Question (b)

Question (i)