Weight of 500 sheets of A4 paper?

That depends upon the density of the paper.

16 sheets of A4 paper make exactly 1 sq m.

Manufacturers make paper in varying thickness as specified by the density in grams per square metre (gsm). The usual densities are 60 gsm, 70 gsm, 80 gsm, 90 gsm. These lead to weights of:

60 gsm → 500 x 60/16 = 1875 g = 1.875 kg = 1 7/8 kg

70 gsm → 500 x 70/16 = 2187.5 g = 2.1875 kg = 2 3/16 kg

80 gsm → 500 x 80/16 = 2500 g = 2.5 kg = 2 1/2 kg

90 gsm → 500 x 90/16 = 2812.5 g = 2.8125 kg = 2 13/16 kg