Answer:
Percent by mass = 40%
Explanation:
Mass of Compound before heating = 3.5g
Mass of compound after heating = 2.1g
Mass of water present = 3.5g - 2.1g = 1.4g
Percent by mass = ( Mass of water / Mass of compound ) x 100
Percentage by mass = (1.4 / 3.5) x 100
Percent by mass = 0.4 x 100
Percent by mass = 40%