Calculating relative formula mass

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Calculating relative formula mass — Free MYP5 Chemistry Practice Questions

1QuestionConcept of relative atomic mass (Ar) from periodic tableAssessment Practice

2 marks~3 minCriterion D

A mining company analyses a copper ore deposit in Chile. The local ore contains a higher proportion of

^{63}

Cu than the global average. The periodic table lists the relative atomic mass of copper as

A_r(\text{Cu}) = 63.55

State what the value

63.55

represents in terms of copper's two naturally occurring isotopes,

^{63}

Cu and

^{65}

Cu. [1]

Explain why using

A_r(\text{Cu}) = 63.55

to estimate the mass of copper extractable from this Chilean deposit may lead to an inaccuracy in the mining company's yield calculations. [1]

Solutions

2QuestionMr calculations for ionic vs covalent compoundsAssessment Practice

6 marks~9 minCriterion C

Data

NaCl — predicted $M_r$	$58.5$	experimental $M_r$ : $58.5$
$\text{CO}_2$ — predicted $M_r$	$44.0$	experimental $M_r$ : $44.0$
MgO — predicted $M_r$	$40.3$	experimental $M_r$ : $39.8$

A student proposes the hypothesis: "The relative formula mass (

M_r

) calculated from atomic masses is equally accurate for ionic and covalent compounds." Mass spectrometry gives the following

Calculate the percentage discrepancy between the predicted and experimental

M_r

values for MgO. [1]

Explain why the experimental

M_r

values for NaCl and

\text{CO}_2

match their predicted values, while MgO shows a discrepancy. [3]

Evaluate whether the experimental evidence supports the hypothesis, and justify whether

M_r

calculations are equally reliable for ionic and covalent compounds. [2]

Solutions

3QuestionMr calculations for ionic vs covalent compoundsAssessment Practice

12 marks~18 minCriterion A

The graph below shows the relationship between relative formula mass (

M_r

) and melting point for a series of ionic and covalent compounds.

Describe the trend shown in the graph for ionic compounds and for covalent compounds. [2]

The

M_r

of magnesium oxide (MgO) is 40 g/mol and the

M_r

of carbon dioxide (CO2) is 44 g/mol. Justify which compound has the higher melting point, referring to bonding and structure. [4]

Consider the two reactions below.

2\text{Mg}(s) + \text{O}_2(g) \rightarrow 2\text{MgO}(s) \quad \text{(Reaction 1)}

2\text{MgO}(s) \rightarrow 2\text{Mg}(s) + \text{O}_2(g) \quad \text{(Reaction 2)}

Evaluate which reaction is more likely to proceed at room temperature. In your answer, refer to the trend identified in part (a), the nature of the ionic lattice in MgO, and the concept of lattice energy. [6]

Solutions

4QuestionMr calculations for ionic vs covalent compoundsAssessment Practice

3 marks~5 minCriterion B

Four ionic compounds are listed below.

Compound	NaCl	MgO	CaCl $_2$	Al $_2$ O $_3$
$M_r$	58.5	40.3	111	102
Melting point (°C)	801	2852	772	2072

Deduce whether a linear relationship exists between

M_r

and melting point, using two specific examples from the data. [1]

Explain why melting point in ionic compounds depends on ionic charge and ionic radius rather than

M_r

. [1]

Analyse why MgO (melting point 2852°C) has a significantly higher melting point than CaCl

_2

(melting point 772°C), despite CaCl

_2

having a much larger

M_r

. [1]

Solutions

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