1) Nice question! The answer is simple, though.
Aq. Soln of sodium carbonate is alkaline in nature b'coz it's a salt formed by the neutralization of a strong base and a weak acid . Similarly, ammonium chloride is formed of a weak base, ammonium hydroxide ,and strong acid, hydrogen chloride making it's aq. Soln slightly acidic
2) Obviously the second soln, ie. Soln. Y will be a stronger acid