If you're not used to Portuguese pronunciation, it can be hard to hear the difference between "al" and "ão" at the ends of words, especially when people speak quickly. We don't have these sounds in English although we have some things vaguely close to them. The robotic voice here (which I agree is quite poor) doesn't help, but...I find that once I was able to hear the distinction between the sounds, I could hear it clearly even with the crappy robot voice.
I found several tools helpful for this. One, Google Voice (which you can access on Google translate) pronounces them more clearly than DuoLingo's robotic voice.
The ã in ão is closer to the short "e" sound in English than the "a" in "al". I find the "ão" dipthong is more distant from English sounds because we generally don't have a "u/oo" sound follow the short "e", whereas the "al" is closer to common sounds in English.
I also like Forvo, it's great for listening to actual humans:
I hope you find this helpful. For me, the sounds initially sounded confusing but then they suddenly "clicked" and now I don't think I could confuse them, even with the shoddy robot voice.
Capital = the city where the goverment is seated (at least usually)
Capitol = the building where the legislature meets
Capital also has the finance/economic aspect, as well as the BIG letters in writing... and then there is capital punishment...
A capital = governing city
O capital = finance/money, stock, fund
Capital = adjective meaning principle, main, central, basic, crucial
acesso ao capital = access to the capital [finance]
longe da capital = far from the capital [city]
uma preocupação capital = a key concern