Very much depends on what kind of MTB you're looking for, and what you're ready to spend on it. A high-end XC(cross country) MTB can be around 20-22 lbs(less than 10 kg). For an FS DH MTB (full suspension downhill) around 35 lbs(15 kg) is more to be expected. Here is a bunch of mountain bike frames so you can see the difference in weight ratios....
Depends on how hard you're riding it. At slow speeds over an even surface 300 kg shouldn't be a problem. But hit a curb at that load and with a bit of speed and you'll probably end up collapsing the wheels.
some do, that's just a little bit lighter than usual, as my hardtail weights 29 lbs
Really hard to tell. A race-ready XC(cross country) can be about 18 lbs, while a heavy-duty DH(downhill) bike can be 40 lbs. Freeride, all-mountain bikes will be somewhere inbetween.
Can be.
A nice road bike will be lighter, a cheap MTB will be heavier.
It is a sport were you ride down a mountain and take jumps on a bike.
IT DEPENDS ON WHAT POCKET BIKE DO U HAVE
take some weight off it and you could fill it with helium
It is not very hard to put full downhill on a normal mountain bike but you need to take it to a professional.
Mountain biking is often done on rough terrain, yes, but it does not have to be. It is a sport that takes place off-road, generally.
1,983,452
About a litre
That depends on your age, your weight, your degree of fitness, your body composition, and where you are cycling (flat ground or incline) as well as how far you go and how fast you go.
Around $200 without taxes
600ml
it depends on the bike standard and the make. Usally the parts are dependent as to how much the bikes worth, and if they are customized, brand new or expensive than you are looking to sell a bike for £200 -£300, but you need to take into consideration the condition of the bike and weather it is up and running.
however you want. just take good car of it.