JEE Main Previous Year Papers Questions of Chemistry With Solutions are available at eSaral. Simulator Previous Years AIEEE/JEE Mains Questions Q. 29.5 mg of an organic compound containing nitrogen was digested according to Kjeldahl's method and the evolved ammonia was absorbed in 20 mL of 0.1 M HCl solution. The excess of the acid required 15 mL of 0.1 M NaOH solution for complete neutralization. The percentage of nitrogen in the compound is :- (1) 29.5 (2) 59.0 (3) 47.4 (4) 23.7 [AIEEE-2010]
Ans. (4) Equation of $\mathrm{NH}_{3}$ $=(0.1 \times 20)-(0.1 \times 15)=0.5$ wt. of $\mathrm{NH}_{3}$ $=0.5 \times 17=8.5 \mathrm{mg}$ wt. of $^{\prime} N^{\prime}$ $=\frac{14}{17} \times 8.5 \mathrm{mg}=7 \mathrm{mg}$ $\%$ of $^{\prime} \mathrm{N}^{\prime}=\frac{7}{29.5} \times 100=23.7$
Q. The recommended concentration of fluoride ion in drinking water is up to 1 ppm as fluoride ion is required to make teeth enamel harder by converting $\left[3 \mathrm{Ca}_{3}\left(\mathrm{PO}_{4}\right)_{2} \cdot \mathrm{Ca}(\mathrm{OH})_{2}\right]$ to : (1) $\left[3\left(\mathrm{CaF}_{2}\right) \cdot \mathrm{Ca}(\mathrm{OH})_{2}\right]$ (2) $\left[3\left(\mathrm{Ca}_{3}\left(\mathrm{PO}_{4}\right)_{2} \cdot \mathrm{CaF}_{2}\right]\right.$ (3) $\left[3\left(\mathrm{Ca}(\mathrm{OH})_{2}\right] \cdot \mathrm{CaF}_{2}\right]$ (4) $\left[\mathrm{CaF}_{2}\right]$ [JEE-Mains -2018]
Ans. (2) $\left[3 \mathrm{Ca}_{3}\left(\mathrm{PO}_{4}\right)_{2} \cdot \mathrm{Ca}(\mathrm{OH})_{2}\right]+2 \mathrm{F}^{\ominus} \longrightarrow$ (drinking water upto $1 \text { ppm })$ $\left[3 \mathrm{Ca}_{3}\left(\mathrm{PO}_{4}\right)_{2} \cdot \mathrm{CaF}_{2}\right]+2 \mathrm{OH}^{\ominus}$ (Harder teeth enamel)
Environmental Chemistry connects with several other topics, so the JEE Mains chapter wise PYQ page is useful for planning broader revision.
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