Tough Interview question - mathematic

If you got 8 balls, one of them has 9.8g and the remainders are 10g,

You are given one balance. What is the minimum try needed for you to find out the 9.8g ball?

Tough Interview question - mathematic 2

You are given one 3 litre container and one 5 litre container.

How to use this two containers to measure 4 litre water accurately and what is it minimum step to achieve it.

Note: There is no any label on the container and it is impossible for you to obtain 1.5 litre water in half full 3 litre container. It is because all of them are in strange shape.