06 The Mole Concept and Stoichiometry, Set A To print: Ctrl + P, paper A4, margins Default.
Learners' AlleyDesigned by Khow Willard
06
The Mole Concept and Stoichiometry
Set AFoundations
Name
Time 1 hour
Total 38 marks
A Data Booklet may be used. Molar volume of a gas = 24.0 dm3 mol−1 at r.t.p.
A
From the lesson
23 marks
1
Bromine has two isotopes, 79Br and 81Br. Ar(Br) = 79.90. Calculate the percentage abundance of 79Br.
[2]
Source: Learners' Alley, The Mole Concept slides
2
CommonA hydrocarbon contains 85.7% carbon by mass and has Mr = 56.0. Determine its molecular formula.
[2]
Source: Learners' Alley, The Mole Concept slides
3
Challenge0.600 g of a compound of carbon, hydrogen and oxygen burns completely to give 0.880 g of CO2 and 0.360 g of H2O. Determine its empirical formula.
[3]
Source: Learners' Alley, The Mole Concept slides
4
5.00 g of CaCO3 is added to 50.0 cm3 of 1.00 mol dm−3 HCl.
CaCO3(s) + 2HCl(aq) → CaCl2(aq) + H2O(l) + CO2(g)
Identify the limiting reagent, and calculate the volume of CO2 formed at r.t.p.
[3]
Source: Learners' Alley, The Mole Concept slides
5
Common20 cm3 of a gaseous hydrocarbon is exploded with 150 cm3 of oxygen (an excess). After cooling to r.t.p., the volume of gas is 110 cm3, and 50 cm3 remains after shaking with excess NaOH(aq). Determine the formula of the hydrocarbon.
[3]
Source: Learners' Alley, The Mole Concept slides
6
Common2.50 g of impure CaCO3 is added to 50.0 cm3 of 1.00 mol dm−3 HCl (an excess). The excess HCl needs 20.0 cm3 of 0.500 mol dm−3 NaOH for neutralisation. Calculate the percentage purity of the CaCO3.
[3]
Source: Learners' Alley, The Mole Concept slides
7
Determine the oxidation number of
(a)
Cr in Cr2O72−
[1]
(b)
S in S2O32−
[1]
(c)
N in NH4+
[1]
Source: Learners' Alley, The Mole Concept slides
8
CommonUse half-equations to construct a balanced equation for the reaction of Cr2O72− with Fe2+ in acid. Cr2O72− is reduced to Cr3+, and Fe2+ is oxidised to Fe3+.
[2]
Source: Learners' Alley, The Mole Concept slides
9
25.0 cm3 of 0.100 mol dm−3 SO32− reacts exactly with 20.0 cm3 of 0.0500 mol dm−3 of an oxidising agent containing one atom of metal M in the +5 oxidation state. The sulfur is oxidised to the +6 state. Determine the final oxidation number of M.
[2]
Source: Learners' Alley, The Mole Concept slides
B
Practice
15 marks
10
CommonWhich statement is correct?
AOne mole of a compound is the amount that contains the same number of atoms as there are in 12.000 g of carbon-12.
BThe relative isotopic mass of lithium-7 = (average mass of all the isotopes of lithium) ÷ (1/12 × mass of one atom of carbon-12).
CThe relative atomic mass of oxygen = (average mass of one atom of oxygen) ÷ (1/12 × mass of one atom of carbon-12).
DThe relative molecular mass of a compound E = (average mass of one atom of E) ÷ (1/12 × mass of one molecule of carbon-12).
CommonLead(IV) oxide, PbO2, is a strong oxidising agent. Under basic conditions, solid PbO2 reacts with solid Cr(OH)3 to form CrO42− and PbO22− ions.
(a)
Write the balanced half-equations, and hence construct an overall equation, for the reaction between PbO2 and Cr(OH)3 under basic conditions.
[2]
A 5.00 g sample containing PbO2 was reacted with excess Cr(OH)3 in alkaline solution, and the resulting solution of CrO42− was made up to 250 cm3. A 25.0 cm3 aliquot was acidified, converting the CrO42− to Cr2O72−, and titrated against 0.150 mol dm−3 Fe2+.
ChallengeIf the dissolved oxygen concentration (DOC) in a river falls below 5 mg dm−3, most fish cannot survive. In the Winkler method, river water is shaken with excess alkaline Mn2+, which oxygen oxidises to a pale brown precipitate of Mn(OH)3.
(a)
The reduction half-equation is O2 + 2H2O + 4e− → 4OH−. Write the oxidation half-equation, and hence construct the overall ionic equation for the reaction between Mn2+ and oxygen under alkaline conditions.
[2]
The Mn(OH)3 is then reacted with excess KI, and the iodine formed is titrated with sodium thiosulfate.
25.0 cm3 of river water treated in this way required 25.00 cm3 of 0.00100 mol dm−3 Na2S2O3. (i) Fill in the blanks with whole numbers: ___O2 ≡ ___Mn(OH)3 ≡ ___I2 ≡ ___Na2S2O3[1] (ii) Calculate the amount of dissolved oxygen in 25.0 cm3 of river water. [1] (iii) Calculate the DOC in mg dm−3, and hence decide whether fish can survive in the river.