Calculating relative formula mass
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Calculating relative formula mass — Free MYP4 Chemistry Practice Questions

1QuestionDifference between molecular mass and formula massConcept Practice
2 marks~3 minCriterion A
The diagram shows a sodium chloride (NaCl\text{NaCl}) crystal lattice and a water (H2O\text{H}_2\text{O}) molecule.
a
State which substance has a molecular mass and which has a formula mass. [1]
b
Explain, with reference to bonding and structure, why different mass terminology is used for each substance. [1]
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Solutions

2QuestionCalculation of relative formula mass (Mr) for compoundsAssessment Practice
3 marks~5 minCriterion C
A graph of mass (g) against number of moles for a pure compound produces a straight line through the origin with a gradient of 44.
a
Using the equation mass=moles×Mr\text{mass} = \text{moles} \times M_r, explain what the gradient of this graph represents. [1]
b
Calculate the MrM_r of the compound. [1]
c
Three compounds share this MrM_r: CO2CO_2, N2ON_2O, and C3H8C_3H_8. Evaluate whether the graph alone is sufficient to identify which compound is present. Justify your answer. [1]

Solutions

3QuestionConcept of relative atomic mass (Ar) from periodic tableAssessment Practice
6 marks~9 minCriterion D
Natural uranium consists of two isotopes: uranium-235 (mass 235.04) and uranium-238 (mass 238.05). The periodic table lists uranium's relative atomic mass (ArA_r) as 238.03. This value is derived from uranium ore samples collected worldwide.
a
Let xx represent the fractional abundance of uranium-235. Show that the percentage abundance of uranium-235 in natural uranium is approximately 0.66%. [2]
b
Explain why a uranium ore sample from a single mine may have a slightly different ArA_r value than the one listed in the periodic table. [2]
c
Evaluate one limitation of using the periodic table's ArA_r value when assessing the radioactivity of a specific uranium ore sample, and identify one environmental risk associated with uranium mining. [2]
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Solutions

4QuestionCalculation of relative formula mass (Mr) for compoundsAssessment Practice
3 marks~5 minCriterion B
The table below shows the molecular formulae and relative formula masses (MrM_r) of four alkanes.

AlkaneMethaneEthanePropaneButane
Molecular formulaCH4CH_4C2H6C_2H_6C3H8C_3H_8C4H10C_4H_{10}
MrM_r16304458
a
State the trend in MrM_r as the number of carbon atoms increases. [1]
b
Each successive alkane differs by one CH2CH_2 unit. Calculate the MrM_r of this CH2CH_2 unit and show how it accounts for the increase in MrM_r at each step. [1]
c
Pentane (C5H12C_5H_{12}) is the next alkane in the series. Using the pattern identified above, deduce its MrM_r and explain whether this value is consistent with the homologous series trend. [1]

Solutions

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