The first thing to understand is what a mole is. It is just a number chemists use to express weight. This many atoms of any element in the periodic table equals its atomic weight. Molecular weight is the sum of atomic weight. Molarity is how chemists express concentration. The units are moles per liter. All weights and volumes in chemistry use the metric system.

The next thing to understand is the definition of pH. It is the negative log of the Hydrogen ion concentration in terms of moles/liter. Both acids each produce a single Hydrogen ion, but their molecular weights are different.

HCl = 1 + 35.5 = 36.5 grams/mole
HNO3 = 1 + 14 + 16*3 = 63 grams/mole

5 grams / (36.5 grams/mole) = 0.137 moles of HCl
5 grams / (63 grams/mole) = 0.079 moles HNO3

0.137 moles / 3.4 liters = 0.040 moles/liter for HCl
0.079 moles / 3.4 liters = 0.023 moles/liter for HNO3

Each mole of acid will produce a mole of Hydrogen ions. This is true for mole fractions as well.

pH of HCl = -LOG(0.040) = 1.39
pH of HNO3 = -LOG(0.023) = 1.63

I would say it was the HCl. The reason the answer was not exact was because the atomic weights were rounded to integers. Try using exact atomic weights and the answer ought to be closer to 1.2.

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