Here are the completed sentences using either "will" future or "going to" future:

1. I think in 50 years cars **will run** on sunlight.
2. What do you need that bucket of water for? I **am going to wash** the car.
3. Jill **is going to come** to the youth club tomorrow. She has promised to give us some tips.
4. Do you expect that he **will visit** us during his stay?
5. It has already **been** 25 degrees; it **is going to be** very hot today.
6. What do you think you **will do** next year at this time?
7. Have you bought some milk? Oh no! I completely forgot; I **am going to go** now and buy it.
8. Have you made up your mind about next year's holidays? Yes, I **am going to go** to my uncle's place in California.
9. **Are you going to ask** her for a second chance?

Feel free to ask if you need more help!