H₂S Partial Pressure Calculator and Gas-Law Formula

An H₂S partial pressure calculator finds hydrogen sulfide’s share of a gas mixture’s total pressure from its mole fraction, or calculates its pressure contribution from the mixture’s composition. Enter the total pressure and the H₂S fraction; the result is the H₂S partial pressure in the same pressure units as the input.

Formula

For an ideal gas mixture, Dalton’s law gives the partial pressure of a component as its mole fraction multiplied by the total pressure:

p_H₂S = y_H₂S × P_total

p_H₂S is the H₂S partial pressure, in kPa here.
y_H₂S is the H₂S mole fraction, a unitless value from 0 to 1.
P_total is the total absolute pressure of the gas mixture, in kPa.

If the composition is reported as a percentage, divide the percentage by 100 to obtain the mole fraction. For example, 1% is 0.01. If reported in parts per million by volume (ppmv), divide ppmv by 1,000,000. The calculator expects a fraction, not a percentage or ppmv value.

Worked Example

Using the calculator’s defaults, the total pressure is 101.325 kPa and the H₂S mole fraction is 0.01, equivalent to 1%.

p_H₂S = y_H₂S × P_total

p_H₂S = 0.01 × 101.325 kPa = 1.01325 kPa

Rounded to three decimal places, the H₂S partial pressure is 1.013 kPa. This is the pressure contribution attributed to H₂S under the ideal-gas mixture model, not the total pressure.

Assumptions

The gas mixture is treated as ideal, so Dalton’s law applies.
The input is the total absolute pressure, not gauge pressure.
The H₂S mole fraction describes the gas mixture at the conditions of interest.
The calculator does not account for chemical reaction, gas dissolution, condensation, or non-ideal gas behavior.

What Partial Pressure Means

Partial pressure is the pressure associated with one gas component in a mixture. In the ideal-gas model, each component contributes in proportion to its share of the total number of gas molecules. The component partial pressures add up to the total pressure:

P_total = p_H₂S + p₁ + p₂ + …

The mole fraction is the amount of H₂S divided by the total amount of gas, with both amounts measured on the same basis. For gases in an ideal mixture, this is equivalent to the volume fraction at the same temperature and pressure. That is why gas composition may be reported as percent by volume or ppmv and converted to mole fraction for this calculation.

Partial pressure is not the same as concentration in a liquid, a mass concentration such as mg/m³, or a chemical activity. Those quantities require additional information, such as temperature, molecular mass, phase behavior, and sometimes a suitable equilibrium model.

How to Use the Calculator

Enter the mixture’s total absolute pressure in kPa. Convert other pressure units before using the result; for example, 1 atm = 101.325 kPa.
Enter the H₂S mole fraction between 0 and 1. Convert a percentage by dividing by 100, or convert ppmv by dividing by 1,000,000.
Read the calculated H₂S partial pressure in kPa. The result uses the same pressure basis as the total-pressure input.

For example, 250 ppmv corresponds to a mole fraction of 250 / 1,000,000 = 0.00025. At 101.325 kPa total pressure, the ideal-gas partial pressure is 0.00025 × 101.325 = 0.02533125 kPa, or about 0.0253 kPa. Keep enough digits during conversion and round only the final reported value.

Pressure Basis, Limits, and Common Errors

Dalton’s law uses absolute pressure. Gauge pressure is measured relative to surrounding atmospheric pressure, so it must not be substituted directly. Convert gauge pressure to absolute pressure by adding the local atmospheric pressure, using consistent units. For instance, a gauge reading of 0 kPa does not mean zero absolute pressure; it means the pressure is approximately atmospheric.

Other frequent mistakes include entering 1 for 1% instead of 0.01, treating ppmv as a fraction, and mixing pressure units. A fraction greater than 1 is invalid for one component of a mixture. A value of 0 means no H₂S is present in the specified mixture, while 1 means the gas is entirely H₂S.

The ideal-gas relationship is a useful approximation for many ordinary gas-mixture calculations, but real-gas interactions can matter at high pressures or under conditions where gases depart substantially from ideal behavior. In those cases, partial pressure may not fully describe the component’s thermodynamic behavior; an equation of state or fugacity-based treatment may be needed. The calculator also does not determine the composition: the H₂S fraction must come from a reliable measurement or stated mixture specification.

Safety and Scope

Hydrogen sulfide is a highly hazardous toxic gas. A calculated partial pressure is not a measure of safe exposure, and this arithmetic is not an exposure-limit assessment, gas-detection method, or substitute for a risk assessment. Do not use the result to infer that a gas mixture is safe to encounter. Any work involving H₂S must be governed by an approved safety plan and qualified oversight; follow your lab method, the safety data sheet (SDS) and your supervisor.

Frequently Asked Questions

How do I calculate H₂S partial pressure?
Multiply the total absolute pressure by the H₂S mole fraction: p_H₂S = y_H₂S × P_total. Use consistent pressure units.

How do I convert H₂S ppmv to partial pressure?
Divide the ppmv value by 1,000,000 to get the mole fraction, then multiply by the total absolute pressure. For example, 250 ppmv is a fraction of 0.00025.

Can I use a percentage instead of a mole fraction?
Convert the percentage to a fraction first: divide by 100. Thus, 0.5% becomes 0.005. Entering 0.5 directly would mean 50%, not 0.5%.

Should I enter gauge or absolute pressure?
Enter absolute pressure. If you have gauge pressure, add local atmospheric pressure after converting both values to the same units.

Is H₂S partial pressure the same as its concentration?
No. Partial pressure describes a gas component’s pressure contribution. A concentration such as ppmv is a composition ratio, while liquid concentration or mass per volume requires additional conversions and assumptions.

Formula used

in pt | Total gas pressure | kPa | 101.325 | 0
in yf | H₂S mole fraction | - | 0.01 | 0 | 1
out ph2s | H₂S partial pressure | kPa | pt * yf | 3

Each line is one input (in, sel, list) or one result (out: label, unit, formula, decimals). How the calculators work

Method: the standard formula for this calculation with the constants and conversion factors written on the page, rounded as each result states. See method and constants, the disclaimer or report an error.

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