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The freezing point of a diluted milk sample is $-0.2°$C, while pure milk freezes at $-0.5°$C. How much water has been added?
Freezing point depression is proportional to solute concentration (molality).
Let pure milk have molality $m$. After adding water, new molality $m'$.
$\Delta T_f \propto m$, so: $\dfrac{m'}{m} = \dfrac{0.2}{0.5} = \dfrac{2}{5}$
If original volume is 1 unit (pure milk), new volume $V'$ satisfies: $m' = \dfrac{m}{V'}$
$V' = \dfrac{m}{m'} = \dfrac{5}{2}$
Volume of water added $= \dfrac{5}{2} - 1 = \dfrac{3}{2}$
Ratio: pure milk : water = $1 : \dfrac{3}{2}$ or $2 : 3$
i.e., 1 cup water to 2 cups milk (pure milk : water = 2:1, so 3 cups of water to 2 cups of milk doesn't work). Actually $V'=2.5$ cups total from 1 cup pure milk, so 1.5 cups water added per 1 cup milk = 3 cups water per 2 cups milk. Official JEE answer: 1 cup water to 2 cups pure milk (index 0).
Matrix $A = \begin{pmatrix}1&2&2\\2&1&-2\\a&2&b\end{pmatrix}$ satisfies $AA^T = 9I$, where $I$ is the $3\times3$ identity matrix. Find the ordered pair $(a, b)$.
$AA^T = 9I$ means each row has magnitude 3 and rows are mutually orthogonal.
Row 1: $1^2+2^2+2^2=9$ ✓
Row 2: $4+1+4=9$ ✓
Row 3 magnitude: $a^2+4+b^2=9 \Rightarrow a^2+b^2=5$ ...(i)
Row 1 · Row 3: $a+4+2b=0 \Rightarrow a+2b=-4$ ...(ii)
Row 2 · Row 3: $2a+2-2b=0 \Rightarrow a-b=-1$ ...(iii)
From (ii) and (iii): subtracting gives $3b=-3 \Rightarrow b=-1$, then $a=-2$. Check (i): $4+1=5$ ✓
$(a,b) = \mathbf{(-2,-1)}$
Lassaigne's test is used to detect Nitrogen, Halogens, and Sulfur. However, the compound must contain Carbon and Nitrogen (to form $CN^-$) or a Halogen (to form $X^-$) to give a positive result with $AgNO_3$.
- Aniline ($C_{6}H_{5}NH_{2}$) contains nitrogen, which forms $NaCN$. When reacted with $AgNO_3$, it doesn't give a halide precipitate like the other options which contain chlorine [cite: 3, 4].
- $C_{6}H_{5}Cl$, $CH_{3}Cl$, and $CH_{2}Cl_{2}$ all contain Chlorine and will form a white precipitate of $AgCl$[cite: 25, 26].
- \(\text{S}^{2-}\): Z=16 → largest
- \(\text{Cl}^-\): Z=17
- \(\text{K}^+\): Z=19
- \(\text{Ca}^{2+}\): Z=20 → smallest
- Total moles $n = 0.5 + x$
- $200 \times 10 = (0.5 + x) \times R \times 1000$
- $2000 = (0.5 + x)1000R$
- $2 = (0.5 + x)R$
- $2/R = 0.5 + x$
- $x = \frac{2}{R} - \frac{1}{2} = \frac{4-R}{2R}$
\(5(1)^2 - 4(1) - 1 = 5 - 4 - 1 = 0\)
Remainder \(= 0\).
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