can you please help?

Question

how do you find the mass of 100 cm of oxygen? how do you find the mass of 200cm of hydrogen?

Answer 1

You would need the density of the gasses and the volume.
I'm assuming you mean cm³ (which is a measure of volume)

density = mass/volume

the density of oxygen is 1.429 g/L, which equals 0.001429 g/cm³

you have the density and volume, so you need to solve for "mass", like in algebra

mass = density x volume

just plug in the numbers and solve

mass = 0.001429 g/cm³ x 100 cm³

mass = 0.1429 g

Answer 2

You probably mean cubic cm (cm^3). In this case, one cubic centimeter is the same as one milliliter (mL). So, you have 100mL of oxygen (0.1L) and 200mL (0.2L) of hydrogen. Now, just multiply these volumes time the density of the element.
Example for oxygen:
0.1L (1.429g/L)= 0.1429g
The above calculation is only true of the standard state, however, because oxygen is a gas, its density changes with temperature and pressure.

Answer 3

Assumption - cm is cm^3
1.0 cm^3 = 1 ml
22,400 ml of O2 at STP 1s 32g
Therefore the mass of 100cm^3 of O2 is
= (100/22,400)x32
= 0.14 g
The mass of 200 cm^3 of H2 is
= (200/22,400)x 2
= 0.018 g

Answer 4

Do you mean 100 cm3 ?

You need to know the pressure and temperature. Then you use

PV = nRT to find n, the number of moles, and multiply by the molar mass.

Check that your units are consistent. T must be the temperature Kelvin, and if R is in L atm mol-1, then obviously V must be in L and P in atm.