Question
20. Calculate the percentage by mass of each of the following component elements in the respective compounds. (a) Sulphur in tetraoxosulphate (VI) acid, H_(2)SO_(4) (b)Phosphorus in tetraoxophosphate (V) acid, H_(3)PO_(4) (c) Oxygen in calcium trate (V) Ca(NO_(3))_(2) 21. Find the empirical formulae of the following compounds from their percentage composition by mass. (a) N=26.17% ;H=7.48% ;Cl=66.35% (b) Ca=71.43% ;O=28.57% (c) Ag=63.53% ;N=8.23% ;O=28.24% (d) Na=32.40% ;O=45.07% ;S=22.53% (N=14,H=1,Cl=35.5,Ca=40,O=16 Ag=108,Na=23,S=32
Answer
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(273 Votes)
Caleb
Master · Tutor for 5 years
Answer
1. For Question 20:(a): The molecular mass of S in H2SO4 is 32g/mol . The molecular mass of H2SO4 is (1*2) + 32 + (16*4) = 98g. Therefore, the %mass of S in H2SO4 = (32/98) * 100% = 32.65%.(b): The molecular mass of P in H3PO4 is 31g/mol. The molecular mass of H3PO4 is (1*3) + 31 + (16*4) = 98g. Therefore, the %mass of P in H3PO4 = (31/98) * 100% = 31.63%.(c): The molecular mass of O in Ca(NO3)2 = (3*16)*2 = 96g/mol. The molecular mass of Ca(NO3)2 = 40 + 14*2 + 96= 164g. Therefore, the %mass of O in CaNO3 = (96/164) * 100% = 58.54%.2. For Question 21:(a): For element N, atom ratio =26.17%/14 ≈ 1.87; For H, atom ratio =7.48%; For Cl, atom ratio = 66.35%/35.5 ≈1.87. So, the empirical formula is NHCl.(b): For element Ca, atom ratio = 71.43%/40 ≈ 1.79; For O, atom ratio =28.57 %/16 ≈ 1.79. So, the empirical formula is CaO.(c): For element Ag, atom ratio =63.53%/108≈ 0.59; For N, atom ratio =8.23%@23/= 0.36; For O, atom ratio =28.24%/16 ≈ 1.76. Taking the ratio with the lowest ratio set as 1, the simplest empirical formula = Ag(N2O6).(d): For element Na, atom ratio = 32.40%/23 ≈ 1.41; For S, atom ratio = 22.53%@/@32= 0.7; For O, atom ratio = 45.07%/16 ≈ 2.82. So, the empirical formula = Na2SO4. .
Explanation
1. For question 20, we need to calculate the percentage mass of various elements in different chemical compounds. - The formula for mass percentage is: (mass of element in compound / total molar mass of compound) * 100% The total molar mass of the compound can be calculated using either a periodic table or tables of atomic masses. The abbreviation “(VI)” or “(V)” signifies the Oxidation States of Sulphur and Phosphorus in the acids H2SO4 and H3PO4, respectively, which play no role in the mass percent calculation. Similarly, in Ca(NO3)2, Ca, N & O all carry certain positive or negative charges-"+", "-". But the charges are not required for solving this problem.2. For question 21 : Given the percentage composition of elements in certain compounds, the empirical formula needs to be found. For these calculations, divide the given percentages of each element by their respective atomic mass, and the quotient received means ratio of no. of atoms of different elements in corresponding compounds. Working through these ratios can then deliver the 'empirical' or 'simplest' ratio in which the elements combine to form these compounds.