Which of the following would be a polar molecule? Why?
HCl - see below
Determining molecular polarity requires 2 things: Bond Polarity and Geometry. Once you have those, you have to figure out if you have a net dipole moment. I think of this kind of like vector addition.
Both atoms of oxygen are strongly electronegative. This is like one really strong person pulling a rope a clone of themselves. They are both super strong and pull super hard, and the rope doesn't move. If nothing moves, its nonpolar.
H has an electronegativity of 2.1, whereas Cl has an electronegativity of 3.0. The difference is 0.9. This is like a strong adult playing a tug-of-war with a school kid. The school kid can still hold the rope, but the adult will pull the kid across the floor. The entire system will move, and so this is Polar.
The bonds in C-H are nonpolar to begin with (2.5 and 2.1 electronegativity, respectively). Also, the bond angles are all 109.5 degrees, so even if there was a pull, it would be symmetric and cancel out.
Same reasoning as above for