Sorry there is no such thing as a free tinned soup, they will all have syns and you can work them out using 1 syn for every 20 calories. Some can be used as a HEB however. Try doing a search as someone has almost certainly asked for low syn soups before.
Yes you can use the 20 calories because pre-prepared soups do not qualify for a free food allowance. They are named in the list of exempted items eg jars of pasta sauces, pre-prepared sandwiches etc. So it is just a straight 1 syn for every 20 calories on tinned soups.