How many molecules of hydrogen gas are present in a 750 ml
container at STP?

1.99 x 10²² molecules H2

Explanation:First we will solve for the moles of H2 using Ideal gas law PV= nRT then derive for moles ( n ).

At STP, pressure is equal to 1 atm and Temperature is 273 K.

Convert volume in mL to L:

750 mL x 1 L / 1000 mL

= 0.75 mL

n = PV/ RT

= 1 atm ( 0.75 L ) / 0.0821 L.atm/ mole.K ( 273 K)

= 3.3x10-² moles H2

Convert moles of H2 to atoms using Avogadro's Number.

3.3x10-² moles H2/ 6.022x10²³ atoms H2 / 1 mole H2

= 1.99x10²² atoms H2

answer: c. the experiment shows that the red substance experienced a chemical change.

explanation: chemical change is the change in which chemical composition of the substance changes. physical change is the change in which there is no change in chemical composition.

chemical changes are accompanied by following changes:

a) precipitate formation

b) energy is released or absorbed

c) formation of bubbles

d) change in color

as it is given that the red color has disappeared and there is production of gas, it means the red substance has undergone a chemical change.

e

let that person have a turn to speak : )