Example 1
P and Q are two elements which forms P2Q3 and PQ2. If 0.15 mole of P2Q3 weights 15.9 g and 0.15 mole of PQ2 weights 9.3 g, what are atomic weights of P and Q?
Solution
Let atomic weight of P and Q a and b respectively
∴ Molecular weight of P2Q3 = 2a + 3b
Potassium selenate is isomorphous with potassium sulphate and contains 45.52% selenium by weight.
Calculate the atomic weight of selenium. Also report the equivalent weight of potassium selenate.
Solution
Potassium seleante is isomorphous to K2SO4 and thus its molecular formula of K2SeO4. Now molecular weight of K2SeO4 = (39 × 2 + a + 4 × 16)
where a is atomic weight of Se (142 + a)g K2SeO4 has Se = ag
Example 3
The molecular mass of an organic acid was determined by the study of its barium salt 4.209 g of salt was quantitatively converted to free acid by the reaction with 21.64 ml of 0.477 M H2SO4. The barium salt was found to have two mole of water of hydration per Ba2+ ion and the acid is mono basic. What is molecular weight of anhydrous acid?
Solution
Meq. of barium salt = meq. of acid 4.290
M / 2×1000 = 21.64 × 0.4777 × 2 Molecular weight of salt = 415.61
Molecular weight of anion = 415.61 137.36
121.31 containing 109.5 gm of the acid per litre for complete neutralization. Calculate the volume of 0.84 N H2SO4 that will be completely neutralized by 125 g of Na2CO3 solution.
Solution
Since Na2CO3 is completely neutralized by HCl
∴ Meq. of Na2CO3 = Meq. of HCl
N × 25 = 32.9 × 3 For neutralization of H2SO4 Meq. of borax = Meq. of H2SO4 (Na2S2O3 . 5H2O) for complete reaction. Calculate mass mixture. The reactions are
As2O3 + 2I2 + 2H2O → As2O5 + 4H+ + 4I¯
As2O5 + 4H+ + 4I¯ → As2O3 + 2I2 + 2H2O Solution
Meq. of I2 used = 20.10 × 0.05 = 1.005
Let Meq. of As2O3 Meq. of As2O5 in mixture be a and b respectively. On addition of I2 to mixture. As2+3 is converted to As2+5.
∴ Meq. of As2O5 = Meq. of I2 to mixture used = 1.005 – Meq. of As5+ formed.
or a = 1.005 ...(1)
After the reaction with I2, mixture contains all the arsenic in +5 oxidation state which is then titrated using KI + hypo. Thus,
Meq. of As2O3 as As+5 + Meq. of As2O5 as As+5 = Meq. of liberated I2 = Meq. of hypo used or a + b = 1.113
248 ×100 or a + b = 4.481 By equations (1) and (2), B = 4.481 – 1.005 = 3.476
∴ Wt. of As2O3 = Meq. Eq.Wt 1.005 198
0.0497g
1000 4 100
× = × =
× and. Wt. of As2O5 = 3.476 230
0.19999g 3 4 1000
× =
×
∴ Wt. of mixture = 0.0497 + 0.1999 = 0.2496 g.
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Exercise Exercise Exercise Exercise ---- IIII
General Type
(Fill in the blanks/ True or False/ Assertion & Reason)
True/False
1. Mass of 3.01 × 1023 molecules of methane is 8 g.
2. AgNO3 + NaCl → AgCl + NaNO3 is a substitution reaction.
3. Exact atomic masses of elements can be determined by applying Dulong and Petit’s law.
4. The vapour density of ozone is 24
5. 18 g of water vapour and 18 g of ice will not contain the same number of molecules.
Fill in the Blanks
1. 3 g of a salt of molecular weight 30 is dissolved in 250 g of water. The molality of the solution is _____
2. The weight of 1 x 1022 molecules of CuSO4.5H2O is _____
*
3. The compound YBa2Cu3O7, which shows superconductivity, has copper in oxidation state _____Assume that the rare earth element yttrium is in its usual + 3 oxidation state.
*
4. In particular concentration of HCN, IO3¯oxidizes I2 according to the reaction IO3¯ + I2 + CN¯ → 5I (CN)2¯ + 3H2OIn this reaction, the molar ratio of IO3¯ : I2 is equal to …….. and the equivalent weight of IO3¯ is molecular wt. divided by ………
*
5. The oxidation number of Cr in CrO5 is ………. .*
6. The oxidation number of S in H2S2O8 and H2SO5 is ………. and ………. respectively.*
7. The oxidation number of S and C in Fe(SCN)2 is ………. and ………. respectively8. Hydrazine (N2H4) acts as reducing agent and evolves N2 gas when reacts with I2. Then the molar ratio of N2H4 to I2 is ………. and the equivalent wt of N2H4 is ………. .
*
9. When Fe(SCN)2 reacts with Ba(MnO4)2 in acidic solution, then Fe(SCN)2 oxidises into Fe+3, SO4+2, NO3¯ and CO3–2. Then the molar ratio of ratio of Fe(SCN)2 to Ba(MnO4)2 is ………. .*
10. In a redox reaction, BrO3– (bromate ion) reduces into Br–, then its molecular weight will be equal to equivalent wt. multiplied by ……. .*
11. When I2 oxidises Na2S2O3 into SO4–2 in basic medium and I2 reduces into I¯, then the molar ratio of I2 to Na2S2O3 is ………. .*
12. For the redox reaction MnO2 + 4HCl → MnCl2 + Cl2 + 2H2O, then n-factor of HCl is ………. .*
13. x M FeS2 has ………. N while it oxidizes into Fe2O3 and SO2 with excess O2.*
14. In redox process HCN oxidizes into CO3–2 and NO3–, So its n-factor is ………. and M is ………. N respectively.Assertion and Reason
R : Because due to charge transfer the one unpaired electron of NO shifts to Fe+2 thereby Fe+2 converts into Fe+.
*
5. A : If the brown ring complex, [Fe(H2O)5NO]SO4 by reacting with HCl converts into FeCl3, NOCl and H2SO4 then the equivalent weight of the complex will be equal to its molecular weight devided by two.R : Because the n-factor of the brown ring complex will be two.
*
6. A : The lowest-oxidation state or oxidation number of N is –3R : Because the lowest oxidation number of non-metal = V.E. is valence electron.
*
7. A : The number of moles of KMnO4 that will be needed to react with 1 mole of HCN (While HCN oxidizes into C3¯ and NO3¯) in acidic medium is 0.5.R : Because the n-factor of KMnO4 in acidic medium is 5 and the n-factor of HCN is 10. So, molar ratio
*
of KMnO4 to HCN is 1 : 2.*
8. A : The oxidation number of “S” in H2S2O8 is 7.R : The maximum oxidation number of S is +6, because the maximum oxidation of element is its no. of valence electron (s) or, group number.
*
9. A : A bottle is labeled as “10 V” of H2O2. so, its percentage strength is 5%.R : % strength of H2O2 is nothing but it is the number of grams of H2O2 in 100 mL solution of H2O2 and it is related with volume strength as under 1 “Vol” of H2O2 = 0.303%.
*
10. A : When a 20 mL of mixture (whose one litre contain x m mole, y m mole and z m mole of NaOH, Na2CO3 and NaHCO3 respectively), is treated with 1 N HCl in presence of methyl orange indicator from the very beginning, then the total used volume of HCl is x 2y z100 100 100 ml