Yes, they work. They are just cans of R-134. Look under the hood, over by the A/C system and find out how much yours is supposed
to take before you charge it.
You do have a leak if you are out of refrigerant, but that doesn't necessarily mean you absolutely need to fix it first. If it's a slow leak,
you may well be able to charge it and have it last all summer. If it leaks out soon after, then you definitely need to find and fix the leak.
Note: If you break open the system, e.g., removing a line or a component while fixing it, then you need to have a vacuum pulled on it
before you recharge.
If it still has some refrigerant, this isn't necessary.
The vacuum is to get moisture out of the system. If there is still a partial charge, no moisture can get in, but it it's totally empty, like if you
cracked it open or it has a MAJOR leak, there will be moisture.