Interesting riddles #3

According to a previous article I would like to apologize for not clarified instructions in solving the riddles. I assure you, it won't happen again.

There were 2 ways to solve the problem with measuring 4L. I will disclose one of them.
Fill the 5L tub and pour it to 3L tub, then empty 3L tub and pour there remaining 2 liters from the first one. Fill 5L tub again and pour a one liter to 3L tub. Now you got 4 liters. :)

The only way to solve the second problem is using equal-arm balance scales. At first you should pick 6 balls, separate them to 3x3 and compare them. Let's imagine the different ball is heavier. If they are equally heavy, then 2 balls remain to compare. If 3 balls are heavier, then you pick 2 of them and compare. Again, if they are equally heavy, then the right one is the third, if not it's the heavier one.
And I bring you the new ones... :)

Problem:
You have died and are in limbo. There are two doors. One door leads to heaven, and the other door leads to hell. There are two guardians, one by each door. One guardian always tells the truth, and the other guardian always lies.
Question:
What one question can you ask a guardian to find out which door leads to heaven?

Problem:
There are 10 prisoners and 10 hats. Each prisoner is assigned a random hat, either black or white, but the number of each color hat is not known to the prisoners. The prisoners will be lined up single file where each can see the hats in front of him but not behind. Starting with the prisoner in the back of the line and moving forward, they must each, in turn, say only one word which must be "black" or "white". If the word matches their hat color they are released, if not, they are killed on the spot. A friendly guard warns them of this test one hour beforehand and tells them that they can formulate a plan where by following the stated rules, 9 of the 10 prisoners will definitely survive, and 1 has a 50/50 chance of survival.
Question:
What is the plan to achieve the goal?