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

The pH IS 9.6

Moles NH3 initially present = 0.0750 L X 0.2 mol/L = 0.015 mol NH3  

Moles HNO3 added = 0.015 L X 0.500 mol/L = 7.5X10^-3 mol HNO3 added  

NH3 + HNO3 --> NH4+ + NO3-  

So, after the addition, the solution contains 7.50X10^-3 mol NH4+ and 9.5X10^-3 mol NH3. The concentrations are:  

[NH4+] = 7.5X10^-3 mol / 0.090 L = 0.0833 M  

[NH3] = 1,5X10^-2mol / 0.090 L = 0.1667 M  

The equilbirium involved is:  

NH3 + H2O <--> NH4+ + OH-  

Kb = [NH4+][OH-]/[NH3] = 1.8X10^-5  

1.8X10^-5 = (0.0833)[OH-]/0.1667  

[OH-] = 3.602 X10^-5  

pOH = 4.44

pH = 14.00 - pOH

     = 9.557 or 9.6