Q4. Find the mass percentage of carbon in urea, NH2CONH2 (C = 12.0, H = 1.0, N = 14.0, O = 16.0 g mol−1).
Molar mass = 60.0 g mol−1, with 12.0 g of carbon per mole, so C = 12.0/60.0 × 100 = 20.0%.
Q5. Acetic acid, CH3COOH, made by Kolbe, has the same molar mass as urea, 60.0 g mol−1. Do the two have the same molecular formula?
No. Acetic acid is C2H4O2 (24.0 + 4.0 + 32.0 = 60.0) and urea is CH4N2O (12.0 + 4.0 + 28.0 + 16.0 = 60.0). Equal molar masses do not mean equal formulas.
Q6. Ammonium cyanate contains carbon, yet it was called an inorganic compound. Why?
It is an ionic salt, NH4+OCN−, obtained from mineral sources, and cyanates, like cyanides and carbonates, are by custom treated as inorganic. The dividing line is custom, not a bond rule: urea, built from the same atoms, has no C–H or C–C bond either, yet it is organic.
Q7. Find the mass percentage of hydrogen in methane, the compound Berthelot made in 1856 (C = 12.0, H = 1.0).
CH4: 12.0 + 4 × 1.0 = 16.0 g mol−1; H = 4.0/16.0 × 100 = 25.0%.
Q8. Give four areas of everyday life in which organic compounds are important.
For example, clothing, fuels, polymers (plastics), dyes and medicines, as well as the molecules of life such as DNA and proteins (NCERT page 256).