ACSEE (Form 6) Chemistry — practice by topic

1. Take 100 g: 87.4 g nitrogen and $100-87.4=12.6$ g hydrogen. 2. Divide by relative atomic masses: nitrogen $\dfrac{87.4}{14}=6.243$, hydrogen $\dfrac{12.6}{1}=12.6$. 3. Divide both by the smaller: nitrogen $1$, hydrogen $\dfrac{12.6}{6.243}=2.02$. 4. The simplest whole-number ratio is therefore $1:2$. 5. The empirical formula is NH₂. Forgetting to subtract from 100 to get the hydrogen percentage is the usual first slip. _Source: NECTA ACSEE 2023 Chemistry 132/1, Question 1: Gases_

1. Two conditions must both hold for a hydrogen bond to form. 2. First, hydrogen must be covalently bonded to a small, highly electronegative atom — nitrogen, oxygen or fluorine. 3. Second, that electronegative atom must carry at least one lone pair, which is what the neighbouring hydrogen is attracted to. 4. Water qualifies because oxygen has two lone pairs; ammonia qualifies because nitrogen has one. 5. Missing the lone-pair condition is why candidates could not explain which molecules hydrogen bond and which do not. _Source: NECTA ACSEE 2023 Chemistry 132/1, Question 3: Chemical Bonding_

1. In o-nitrophenol the nitro group sits next to the hydroxyl group on the ring. 2. They are close enough to bond to each other within the same molecule, which is intramolecular hydrogen bonding. 3. In p-nitrophenol the two groups sit opposite each other, too far apart to reach within one molecule. 4. So p-nitrophenol bonds instead to neighbouring molecules, which is intermolecular hydrogen bonding. 5. Writing 'intrahydrogen' and 'interhydrogen' instead of intramolecular and intermolecular was a recorded wording error. _Source: NECTA ACSEE 2023 Chemistry 132/1, Question 3: Chemical Bonding_