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Chemistry Assertion (A) & Reason (B) MCQ

MODEL PAPER 6 · Rajasthan Board (RBSE) STD 12 Science (Rajasthan - English Medium). 4 questions.

Questions

Assertion (A) & Reason (B) MCQ

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4 questions · self-marked practice — reveal the answer and mark yourself.

Question 11 Mark
Assertion (A): Mg is not present in the enamel of human teeth.
Reason (R): Mg is an essential element for the biological functions of humans.
Answer
(b) Both A and R are true but R is not the correct explanation of A.
Explanation: Enamel is the hardest substance in the human body and contains the highest percentage of minerals, with water and organic material composing the rest. The primary mineral is hydroxyapatite, which is crystalline calcium phosphate. Magnesium is not present in human enamel. However, magnesium is an essential mineral for human health. Magnesium is a cofactor for more than 300 enzyme systems and is involved in both aerobic and anaerobic energy generation and in glycolysis, either directly as an enzyme activator or as part of the magnesium-ATP complex. Magnesium is required for mitochondria to carry out oxidative phosphorylation. It plays a role in regulating potassium fluxes and in the metabolism of calcium.
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Question 21 Mark
Assertion (A): Formaldehyde cannot be prepared by Rosenmund's reduction.
Reason (R): Acid chlorides can be reduced into aldehydes with hydrogen in boiling xylene using palladium or platinum as a catalyst supported on barium sulphate. This is known as Rosenmund's reduction.
Answer
(b) Both A and R are true but R is not the correct explanation of A.
Explanation: HCHO cannot be prepared by Rosenmund's reduction because formyl chloride is unstable at room temperature.
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Question 31 Mark
Assertion (A): Boiling point of alkyl halides increases with an increase in molecular weight.
Reason (R): Boiling point of alkyl halides is in the order RI > RB > RCI > RF.
Answer
(a) Both A and R are true and R is the correct explanation of A.
Explanation: The greater the molecular mass, the stronger the van der Waals' forces of attraction and hence higher is the melting point/boiling point.
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Question 41 Mark
Assertion $(A):$ With $\ce{Br_2 H_2O}$, phenol gives $2,4,6-$tribromophenol but with $Br_2 - CS_2$ it gives $4-$bromophenol as the major product.
Reason $(R):$ In water ionization of phenol is enhanced but in $CS_2,$ it is greatly suppressed
Answer
Both $A$ and $R$ are true and $R$ is the correct explanation of $A.$
Both assertion and reason are true and reason is the correct explanation of assertion.
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