Answer:
Although hydrogen has the highest calorific value, it is not used as a domestic fuel. It is because hydrogen is a highly combustible and it reacts explosively when it comes in contact with air. And hence as a result, storing of the hydrogen gas is difficult and is dangerous at the same time.
A. The number of valence electrons increases as atomic mass increases. == Generally true for the representative elements since atomic mass generally increases with increasing Z.
B. The reactivity of alkali metals increases as atomic mass increases. == True. Atomic mass increases down the column and so does reactivity
C. The reactivity of the halogens increases as atomic mass increases. == False. Reactivity decreases down the column.
D. The number of valence electrons decreases across a period. == False. In general, the number of valence electrons increases across a period, particularly for the representative elements.
Each mole of barium sulfate releases equal moles of barium ions and sulfate ions. Let the concentration of one substance be x. Thus:
Ksp = [Ba⁺²][SO₄⁻²]
1.1 x 10⁻¹⁰ = (x)(x)
x = 1.05 x 10⁻⁵
The answer is A.