Exam code: 5070
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3.2 Relative Masses of Atoms & Molecules
Define relative atomic mass, Ar.
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i) State where the relative atomic mass of an element can be found.
[1]
ii) The relative atomic mass of magnesium is 24 and the relative atomic mass of helium is 4.
Calculate how many times heavier one atom of magnesium is than one atom of helium.
[1]
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i) Mr is used as the symbol for relative molecular mass.
Define relative molecular mass.
[1]
ii) State the term that is used in place of relative molecular mass when referring to an ionic compound.
[1]
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i) Calculate the relative molecular mass of carbon dioxide, CO2.
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ii) Calculate the relative molecular mass of ammonia, NH3.
[1]
iii) Magnesium oxide has the formula MgO.
Calculate its Mr.
[1]
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Calcium oxide is an ionic compound with the formula CaO.
Calculate its relative formula mass.
(Ar: Ca = 40, O = 16)
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Calcium burns in oxygen to produce calcium oxide.
2Ca + O2 → 2CaO
In one experiment, 120 g of calcium burned completely in 48 g of oxygen.
i) Calculate the mass of calcium oxide formed.
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ii) In a second experiment, 60 g of calcium burned completely.
Calculate the mass of oxygen used.
[1]
iii) Deduce the mass of calcium oxide formed in the second experiment.
[1]
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Calculate the relative formula mass of magnesium chloride, MgCl2.
(Ar: Mg = 24, Cl = 35.5)
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Explain why the relative molecular mass of chlorine gas is not 35.5.
(Ar: Cl = 35.5)
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Phosphorus forms simple molecules with the formula P4.
Calculate the relative molecular mass of a phosphorus molecule.
(Ar: P = 31)
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The relative atomic mass of chlorine is 35.5.
Explain why this value is not a whole number.
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Ethanoic acid has the formula CH3COOH.
Calculate its relative molecular mass.
(Ar: H = 1, C = 12, O = 16)
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Calculate the relative formula mass of calcium hydroxide, Ca(OH)2.
(Ar: H = 1, O = 16, Ca = 40)
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State why relative formula mass, rather than relative molecular mass, is used for calcium hydroxide.
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Calculate the relative formula mass of aluminium oxide, Al2O3.
(Ar: O = 16, Al = 27)
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Ammonium sulfate contains ammonium ions, NH4+, and sulfate ions, SO42-.
i) Deduce the formula of ammonium sulfate.
[1]
ii) Calculate the relative formula mass of ammonium sulfate.
[2]
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Hydrated sodium carbonate has the formula Na2CO3·10H2O.
i) Calculate its relative formula mass.
[2]
ii) A student writes the answer to part i) followed by the unit g.
Explain why no unit should be written.
[1]
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Barium nitrate has the formula Ba(NO3)2.
Calculate its relative formula mass.
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Sodium burns in chlorine to make sodium chloride.
2Na + Cl2 → 2NaCl
i) Calculate the relative molecular mass of chlorine, Cl2.
[1]
ii) Sodium chloride is an ionic compound.
Calculate its relative formula mass.
[1]
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In one reaction, 92 g of sodium burns completely in chlorine.
i) Calculate the mass of sodium chloride made.
[2]
ii) Deduce the mass of chlorine that reacted.
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A second reaction makes 58.5 g of sodium chloride.
Calculate the mass of sodium that burned.
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An oxide of a metal, X, has the formula X2O3.
The relative formula mass of this oxide is 160.
i) Calculate the relative atomic mass of X.
[2]
ii) Use the Periodic Table to identify metal X.
[1]
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Metal X forms a second oxide with a relative formula mass of 72.
Deduce the formula of this second oxide.
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Calculate the relative formula mass of the chloride of X, which has the formula XCl3.
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Anhydrous copper(II) sulfate has the formula CuSO4.
Calculate its relative formula mass.
(Ar: O = 16, S = 32, Cu = 64)
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Hydrated copper(II) sulfate has the formula CuSO4·5H2O.
Calculate its relative formula mass.
(Ar: H = 1, O = 16, S = 32, Cu = 64)
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Calculate the percentage by mass of water in CuSO4·5H2O.
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A different hydrated salt has the formula ZnSO4·xH2O.
Its relative formula mass is 287.
Deduce the value of x.
(Ar: H = 1, O = 16, S = 32, Zn = 65)
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A compound of an element E with hydrogen has the formula EH4.
The compound contains 75.0% by mass of E.
(Ar: H = 1)
i) Calculate the relative molecular mass of EH4.
[2]
ii) Deduce the relative atomic mass of E.
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iii) Use the Periodic Table to identify E.
[1]
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A second compound of E with hydrogen has the formula E2H6.
Calculate its relative molecular mass.
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Calculate the percentage by mass of hydrogen in E2H6.
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A sample of silicon contains three isotopes.
Of the atoms in the sample, 92.0% are 28Si, 4.7% are 29Si and 3.3% are 30Si.
i) Calculate the relative atomic mass of silicon in this sample, correct to one decimal place.
[2]
ii) Relative atomic masses are often not whole numbers.
Explain why.
[1]
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Use the relative atomic masses given in the Periodic Table.
i) Calculate the relative formula mass of barium hydroxide, Ba(OH)2.
[2]
ii) An oxide of nitrogen has a molecular formula in which the number of oxygen atoms is twice the number of nitrogen atoms.
The relative molecular mass of this oxide is 92.
Deduce its molecular formula.
[2]
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Ammonium carbonate has the formula (NH4)2CO3.
i) Calculate its relative formula mass.
[2]
ii) Calculate the percentage by mass of the element nitrogen in ammonium carbonate.
[2]
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i) In iron(III) hydroxide the ions present are Fe3+ and OH-.
Deduce the formula of iron(III) hydroxide.
[1]
ii) Chromium(III) oxide contains Cr3+ ions and O2- ions.
Deduce its formula.
[1]
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Use the relative atomic masses given in the Periodic Table.
i) Calculate the relative formula mass of iron(III) hydroxide.
[2]
ii) Calculate the relative formula mass of chromium(III) oxide.
[2]
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Cobalt(II) chloride forms hydrated crystals with the formula CoCl2·xH2O.
These crystals have a relative formula mass of 238.
Deduce the value of x.
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The hydrated crystals are warmed until all of the water of crystallisation has been driven off, leaving anhydrous cobalt(II) chloride, CoCl2.
A 23.8 g sample of the crystals is treated in this way.
Calculate the mass of anhydrous cobalt(II) chloride that remains.
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An element, E, forms an oxide with the formula E2O5.
The relative molecular mass of this oxide is 142.
i) Calculate the relative atomic mass of E.
[2]
ii) Use the Periodic Table to identify element E.
[1]
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Element E forms two chlorides.
i) Calculate the relative molecular mass of ECl3.
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ii) The other chloride of E has a relative molecular mass of 208.5.
Deduce the formula of this chloride.
[2]
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Element E also forms a second oxide, which contains only E and oxygen.
This second oxide contains 56.4% of E by mass and has a relative molecular mass of 220.
Deduce the empirical formula and the molecular formula of this second oxide.
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A metal M forms a sulfate with the formula M2(SO4)3.
The relative formula mass of this sulfate is 342.
(Ar: O = 16, S = 32)
i) Calculate the total relative mass of the three sulfate groups in M2(SO4)3.
[1]
ii) Deduce the relative atomic mass of M.
[2]
iii) Use the Periodic Table to identify M.
[1]
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The sulfate forms hydrated crystals with the formula M2(SO4)3·xH2O.
The relative formula mass of the hydrated crystals is 666.
Deduce the value of x.
(Ar: H = 1, O = 16)
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Calculate the percentage by mass of water in the hydrated crystals, M2(SO4)3·xH2O.
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The alkanes are a family of hydrocarbons with the general formula CnH2n+2.
(Ar: H = 1, C = 12)
i) Show that the relative molecular mass of an alkane is given by 14n + 2.
[2]
ii) An alkane has a relative molecular mass of 72.
Deduce the value of n.
[2]
iii) Give the molecular formula of this alkane.
[1]
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A different family of hydrocarbons has the general formula CnH2n.
One member of this family has a relative molecular mass of 84.
Deduce the molecular formula of this hydrocarbon.
(Ar: H = 1, C = 12)
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Use your answer to part (a) iii) to calculate the percentage by mass of carbon in that alkane.
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