21. One mole of K2Cr2O7 has thrice the number of ions than the number of formula units when ionized.
K2Cr2O7 ionize as:
K2Cr2O7 ⇌ 2K1+ + Cr2O7-2
This equation shows that 1 mole of K2Cr2O7, produces 2 moles of K1+ and 1 mole of Cr2O7-2. Hence total of three moles of ions are produced by the ionization of 1 mole of K2Cr2O7
22. 4.9g of H₂SO4 when completely ionized in water has an equal number of positive and negative charges but the number of positively charged ions are twice the number of negatively charged ions.
Sulphuric acid ionizes as follows.
H₂SO4 ⇌ 2H++SO42-
This equation shows that 1 mole of H₂SO4 produces 2 moles of positive ions and 1 mole of negative ions. But the number of charges are the same. Hence whatever the amount of H₂SO4 is, the number of positive ions will be twice the number of negative ions.
23. 180g of glucose and 342g of sucrose have the same number of molecules but the different numbers of atoms present in them. Explain?
| Mass of glucose | = 180 g |
| Number of moles of glucose | = (Mass of glucose) / (Molar mass of glucose) = 180/180 =1 mole |
| Mass of sucrose | = 342 g |
| Number of moles of sucrose | = (Mass of sucrose) / (Molar mass of sucrose) =342/342 =1mole |
And one mole of each compound contains 6.02×1023 numbers of particles. So, both have the same number of particles. Glucose (C6H12O6) has 24 atoms. While sucrose (C12H22O11) has 45 atoms. So equal number of molecules of glucose and sucrose will have a different number of atoms.
24. Calculate the number of CO2 molecules in 20.0 grams of it?
| No. of CO2 molecules (N) | =? |
| Given mass (m) | = 20g |
| The molar mass of CO₂ | = 44g/mole |
| Avogadro’s number (NA) | = 6.022 x 1023 |
| Formula N | = m/M × NA |
| =20/44 × 6.022 ×1023 | |
| No. of CO₂ molecules | = 2.73×1023 |
Molar volume: The volume occupied by one mole of an ideal gas at standard temperature and pressure is called molar volume. Its value is 22.414dm3 e.g. 1 mole of H2 = 6.02×1023 molecules = 22.414 dm3 at S.T.P.
Molecular formula: The formula which shows the exact number of atoms of each element present in one molecule of a compound is called molecular formula. e.g. Benzene = C6H6 and Glucose = C6H12O6
26. What is the function of 50% KOH and Mg (CIO4)2 in combustion analysis?
Mg(CIO4)2 and KOH are used as absorbers in combustion analysis. The water vapours are absorbed by Mg(CIO4)2 and CO2 is absorbed by 50% KOH.
27. What is the molecular ion? Give an example. (Chemistry notes)
Molecular ion: The ion formed due to the loss or gain of electrons by a molecule is called a molecular ion. Example CH4+, N2+, CO+ etc.
28. No individual neon atom in the sample of the element has a mass of 20.18amu explain.
A sample of neon consists of: 
The average of atomic mass is calculated as:
Average atomic mass = (20×90.92%) +(21×0.26%) +(22×8.82%) / 100 = 20.18 a.m.u.
29. Define Avogadro’s number. Give its numerical value.
Avogadro’s number: The number of particles (atoms, ions, molecules or formula units) present in one mole of a substance is called Avogadro’s number. Its value is 6.02×1023.
30. Differentiate between empirical and molecular formulas?
| Empirical Formula | Molecular Formula |
| 1. It shows the simplest whole-number ratio between atoms of a compound. 2. Both covalent and ionic compounds have the empirical formula. e.g. Benzene has an empirical formula as (CH) and glucose has CH2O | 1. It shows the exact number of atoms of each element present in a compound. 2. Ionic compound does not have Molecular formula e.g. Benzene has molecular formula as (C6H6) and glucose has C6H12O6 |
