Question

The following hydrogen halides ($HF$, $HCl$, $HBr$ and $HI$) are all polar molecules. The strength of the acid each forms in water is based on which of the following?
$I$. The polarity of the molecule
$II$. The size of the molecule
$III$. The strength of the bond
A. $I$ only
B. $II$ only
C. $III$ only
D. $I$ and $III$
E. $II$ and $III$

Answer 1

Hint: Acidity of a molecule is its tendency to lose the ${H^ + }$ ions. So, the properties of the molecules which affect the bonding between atoms and hydrogen will affect the acidity of the molecule.

Complete step by step answer:
Acidity of a molecule is measured by its ability to lose the hydrogen ions.
 If the molecule has loosely bound hydrogen ions, it will lose the ions pretty easily as compared to those molecules which are very tightly bound hydrogen ions. So, the molecule with the weakest bond strength will be the strongest acid.
As we look into the 17th column in the periodic table, it contains all the halogens, from $F$ to $I$ from top to the bottom of the 17th group. As we are aware of the fact that the size of the atom increases down the group because of increment in the number of electrons and the shells, the sizes of the atom will be in order $F < Cl < Br < I$. They will also follow the same order when their radiuses are compared.
Also, we know that the $H$ atom has a very small radius and is accounted for being one of the smallest atoms in the periodic table.
Polarity of a molecule is having poles. This means the charge or the attractive force on one atom is more than the other atoms present in the molecule, which make them incapable to cancel out with each other. Higher is the difference between electronegativity, the more polar the molecule.
The electronegativity of atoms decreases down the group, in a periodic table. Fluorine, as we know, is the most electronegative atom in the table. As we go down the group, the size of the atoms also increases. So when the molecules are allowed to dissociate in water, the smaller atoms will be unable to do so as they have stronger bonds. So they will be unable to lose the electrons. Another reason is the stability of the atom after disassociation. Smaller atoms will have larger charge density, being more electronegative, so they will be unstable. In comparison, larger atoms will handle the charge more stably. So the acidic character does depend on the polarity and electronegativity.
Considering the size of individual atoms, the bond length of the molecules will follow the order \[HF < HCl < HBr < HI\]. This is because the radius of Iodine is the largest and to form a molecule, the hydrogen atom will be farthest as compared to all other compounds. Similarly, the molecule with Fluorine, will have the shortest bond because their atom sizes are the smallest, making them attach very closely.
The relation between bond length and bond strength is of inverse nature. So, if the bond length is maximum for $HI$, it will have the least bond strength. So the order for bond strength if \[HF > HCl > HBr > HI\].
With the definition of acidity, stated above, it can be surely told that the order of acidity is \[HF < HCl < HBr < HI\]. So acidity does depend on the strength.
The size of the molecule does not really matter for acidity but the size of individual ions it comprises does. The reasons for the same are stated above.

So, options $I$ and $III$ correct. This makes option D correct.

Note:
 In solving questions about acidity and basicity, one needs to know the basic trends of electronegativity, size, radius, electrons and charges. We also need to keep in mind the ease of dissociation of the molecule into the products and the after state of atoms after dissociation so as to determine the feasibility of the reaction of hydrogen removal. If the atoms obtained after the dissociation are unstable the reaction will not occur.