CourseCourseConceptConceptDocumentDocument

CAIE A-Level Chemistry AS 2.3 Formulas Questions

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

Syllabus
2028–2030
Course
Chemistry 9701
Level
AS

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 numbers2.3.2—Balanced and ionic equations2.3.3—Terms: empirical and molecular formula2.3.4—Terms: anhydrous, hydrated and water2.3.5—Empirical and molecular formulas

Question 1

[Maximum number: 1]

Which pair of formulae is correct?

A

Ag2CO3\mathrm{Ag}_{2} \mathrm{CO}_{3} and (NH4)3NO3\left(\mathrm{NH}_{4}\right)_{3} \mathrm{NO}_{3}

B

K2HCO3\mathrm{K}_{2} \mathrm{HCO}_{3} and Zn3(PO4)2\mathrm{Zn}_{3}\left(\mathrm{PO}_{4}\right)_{2}

C

AgHCO3\mathrm{AgHCO}_{3} and K3PO4\mathrm{K}_{3} \mathrm{PO}_{4}

D

ZnCO3\mathrm{ZnCO}_{3} and (NH4)2PO4\left(\mathrm{NH}_{4}\right)_{2} \mathrm{PO}_{4}

Question 2

[Maximum number: 1]

Three equations are shown.

1CH3COOH+MgCO3→CH3COOMg+CO2+H2O23CH3CH2COOH+Na→CH3CH2COONa+12H23CH3CH2COOH+2Ba(OH)2→CH3CH2COOBa2+2H2O\begin{aligned} & 1 \mathrm{CH}_{3} \mathrm{COOH}+\mathrm{MgCO}_{3} \rightarrow \mathrm{CH}_{3} \mathrm{COOMg}+\mathrm{CO}_{2}+\mathrm{H}_{2} \mathrm{O} \\ & 2 \\ & 3 \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{COOH}+\mathrm{Na} \rightarrow \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{COONa}+\frac{1}{2} \mathrm{H}_{2} \\ & 3 \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{COOH}+2 \mathrm{Ba}(\mathrm{OH})_{2} \rightarrow \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{COOBa}_{2}+2 \mathrm{H}_{2} \mathrm{O} \end{aligned}

Which of the equations are correct?

A

1, 2 and 3

B

1 and 2 only

C

2 only

D

3 only

Question 3

[Maximum number: 1]

Which formula represents the empirical formula of a compound?

A

C2H4O\mathrm{C}_{2} \mathrm{H}_{4} \mathrm{O}

B

C2H4O2\mathrm{C}_{2} \mathrm{H}_{4} \mathrm{O}_{2}

C

C6H12\mathrm{C}_{6} \mathrm{H}_{12}

D

H2O2\mathrm{H}_{2} \mathrm{O}_{2}

Question 4

[Maximum number: 3]

Species such as NH4+,CO32−\mathrm{NH}_{4}{ }^{+}, \mathrm{CO}_{3}{ }^{2-} and PO43−\mathrm{PO}_{4}{ }^{3-} are examples of molecular ions.

There are many naturally occurring hydrated compounds that contain the anion PO43−\mathrm{PO}_{4}{ }^{3-}.

Struvite is a soft hydrated mineral with Mr=245.3M_{\mathrm{r}}=245.3. The anhydrous form of the mineral has the formula NH4MgPO4\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 =

Question 5

[Maximum number: 1]

Hydrated cobalt(II) sulfate loses water when heated to give anhydrous cobalt(II) sulfate. All the water of crystallisation is lost to the atmosphere as steam.

When 3.10 g of hydrated cobalt(II) sulfate, CoSO4⋅×H2O\mathrm{CoSO}_{4} \cdot \times \mathrm{H}_{2} \mathrm{O}, is heated to constant mass the loss in mass is 1.39 g .

What is the value of x, to the nearest whole number?

A

4

B

6

C

7

D

11

All question bank results loaded