How much does an army tank weigh in kg?

Would've been a better question if you'd specified which particular tank you had in mind. There are light, medium, and heavy tanks.

As far as heavy tanks go (which are often referred to as Main Battle Tanks), combat (loaded) weights for a few include:

• M1 Abrams: 54,545 kg
• M60A3 Patton: 52,617 kg
• Challenger 2: 62,500 kg
• Hyundai Type 88: 51,000 kg
• T-72: 44,500 kg
• T-80: 42,500 kg
• C1 Ariete: 54,000 kg

Just a few examples to give you an idea of the weight range for a Main Battle Tank. Source of the information was Jane's Tank Recognition Guide, 1996, ISBN 0-00-470995-0.