STP is a pressure of 1 bar and a temperature of 0 °C.
At STP the volume of 1 mol of any gas is 22.71 L.
So, we can find the moles of each gas and then calculate the volumes.
Hydrogen
#"Moles" = 6.50 color(red)(cancel(color(black)("g"))) × "1 mol"/(2.016 color(red)(cancel(color(black)("g")))) = "3.224 mol"#
#"Volume" = 3.224 color(red)(cancel(color(black)("mol"))) × "22.71 L"/(1 color(red)(cancel(color(black)("mol")))) = "73.22 L"#
Nitrogen dioxide
#"Moles" = 6.50 color(red)(cancel(color(black)("g"))) × "1 mol"/(46.01 color(red)(cancel(color(black)("g")))) = "0.1413 mol"#
#"Volume" = 0.1413 color(red)(cancel(color(black)("mol"))) × "22.71 L"/(1 color(red)(cancel(color(black)("mol")))) = "3.208 L"#
Carbon monoxide
#"Moles" = 6.50 color(red)(cancel(color(black)("g"))) × "1 mol"/(28.01 color(red)(cancel(color(black)("g")))) = "0.2321 mol"#
#"Volume" = 0.2321 color(red)(cancel(color(black)("mol"))) × "22.71 L"/(1 color(red)(cancel(color(black)("mol")))) = "5.270 L"#
Helium
#"Moles" = 6.50 color(red)(cancel(color(black)("g"))) × "1 mol"/(4.003 color(red)(cancel(color(black)("g")))) = "1.624 mol"#
#"Volume" = 1.624 color(red)(cancel(color(black)("mol"))) × "22.71 L"/(1 color(red)(cancel(color(black)("mol")))) = "36.88 L"#
Total volume
The total volume of the gases is
#V = "(73.22 + 3.208 + 5.270 + 36.88) L" = "118.6 L"#
Gas Pressure
All gases are at STP, so they each exert the same pressure: 1 bar.
