Chemical Formulae MCQs

Q1. Why is the formula of magnesium nitride Mg3N2?

• A.Mg has valency 2 and N has valency 3, which cross over
• B.Mg has valency 3 and N valency 2
• C.both have valency 1
• D.nitrogen has no valency

Answer: A. Mg has valency 2 and N has valency 3, which cross over

Given 3, 2, asked for the unknown. By v = u + a*t, F = m*a, PE = m*g*h, n = m/M. hence the required answer = Mg has valency 2 and N has valency 3, which cross over. the stem provides numerical data: 3, 2. Thus option A) Mg has valency 2 and N has valency 3, which cross over. Others fail: option B) 'Mg has valency 3 and N valency 2' is incorrect: Mg's valency is 2 and N's is 3; crossing gives Mg3N2; option C) 'both have valency 1' fails since If both were 1 the formula would be MgN, not Mg3N2.

Q2. Which formula is written correctly for a neutral compound of Ca^2+ and NO3^- (nitrate)?

• A.Ca(NO3)2
• B.CaNO3
• C.Ca2NO3
• D.Ca(NO3)

Answer: A. Ca(NO3)2

We are told that 2, 3 (chemistry, chapter 'Atomic Foundations of Matter'). What we must find: the requested quantity. The principle that connects these is - v = u + a*t, F = m*a, PE = m*g*h, n = m/M. The arithmetic is: hence the required answer = Ca(NO3)2. The stem provides numerical data: 2, 3. That lands on option A) Ca(NO3)2. As for the others, option B) 'CaNO3' doesn't hold - One Ca^2+ needs two NO3^- to balance charge: Ca(NO3)2; option C) 'Ca2NO3' is incorrect: Calcium is the 2+ ion; it takes two nitrates, Ca(NO3)2, not Ca2NO3.

Q3. Why is the formula of water H2O and not HO2 or H2O2?

• A.hydrogen has valency 1 and oxygen valency 2, giving two H to one O
• B.oxygen has valency 1 and hydrogen valency 2
• C.both have valency 2
• D.the formula is chosen arbitrarily

Answer: A. hydrogen has valency 1 and oxygen valency 2, giving two H to one O

Given 2, 2, 2, 2, asked for the unknown. By v = u + a*t, F = m*a, PE = m*g*h, n = m/M. hence the required answer = hydrogen has valency 1 and oxygen valency 2, giving two H to one O. the stem provides numerical data: 2, 2, 2, 2. Hence option A) hydrogen has valency 1 and oxygen valency 2, giving two H to one O. Others fail: option B) 'oxygen has valency 1 and hydrogen valency 2' misses the point - It is the reverse: H is 1, O is 2, giving H2O; option C) 'both have valency 2' misses the point - Hydrogen's valency is 1; the formula is H2O.