Jam contains a high proportion of water, so it would depend on the physical strength of the jar. I'm assuming that this is a glass jar? Most likely, the jar would crack or shatter as the pressure around it was decreased, allowing the water in the jam to boil off into the vacuum.
A jam jar.
About 390 in most.
If the jar was not thick enough to resist the exterior pressure, it would implode, just as a pressurized jar in space would explode.
Jam, jelly, juice, jalapeno peppers, jambalaya, jars, jerk seasonings, jerky chicken or beef, and jumping children!
Check for jam jar labels in the canning section of any hardware or grocery store.
The contents of the jam jar is usually around 145 grams (i.e. not including the weight of the jar). This is usually because (although jam is a preserve, when it is made industrially it is not boiled to the extent that it would be when homemade, so is less "preserved" than it used to be), jam is not as long-life as it used to be. It is advised that it is kept in the fridge, so a 145g jar is usually the right size to fit in the fridge door.
That is a 4 oz jar of jam.
It depends upon the material of which the jar is made. If it is made of glass, as virtually all jam jars are, it is not biodegradable.
The weight of an empty jam jar can vary depending on its size and material. On average, a small empty jam jar may weigh around 100-200 grams, while a larger jar may weigh 300-500 grams.
I think jam is sold by mass/weight rather than volume, but if you want to measure the volume, a typical jam/jelly jar is less than 1 liter, so use milliliters to measure volume.
The helium, being lighter than air, would simply rise out of the jar.
The amount of space in an object is, presumably, the volume of the cavity.