Converting Grains to PPM

I'm having trouble converting a specification we have here at work.

It states that we can have no more than 1/4 grain of H2S per 100 standard cubic ft. (I'm talking about Natural Gas in this case)

I need to convert that number into ppm.

Someone please help.

Thanks

A grain is 1/7000th of a pound, so that is a weight.

You need to decide whether you need you ppm as a ratio of weights, volumes (better suited) to liquids), moles, etc.

If you find the weight of 100 standard cu ft of gas, you already have the allowable weight of H2S, so you can express it as wt/wt.

For a gas, mol/mol and pressure/pressure ratios are the saem. You could aslo use the ideal gas law to determine the partial pressure of the that mole fraction H2s when occupying 100 cu ft at standard temp. The rest of the standard condition pressure is natural gas. Form the ratio.

(Sorry, but I'm metric. I have no idea what the gas constant is in English units)

A grain is 1/7000th of a pound, so that is a weight.

You need to decide whether you need you ppm as a ratio of weights, volumes (better suited) to liquids), moles, etc.

If you find the weight of 100 standard cu ft of gas, you already have the allowable weight of H2S, so you can express it as wt/wt.

For a gas, mol/mol and pressure/pressure ratios are the saem. You could aslo use the ideal gas law to determine the partial pressure of the that mole fraction H2s when occupying 100 cu ft at standard temp. The rest of the standard condition pressure is natural gas. Form the ratio.

(Sorry, but I'm metric. I have no idea what the gas constant is in English units)