TheAlgorithms/Python · error · Exception

Molar mass should be greater than 0 kg/mol

Error message

Molar mass should be greater than 0 kg/mol

What it means

Raised by physics/speeds_of_gas_molecules.py:avg_speed_of_molecule when molar_mass <= 0. Molar_mass appears in the denominator of sqrt(8RT/(pi*M)); zero divides by zero and a negative value yields a complex speed. Like the sibling checks in this file it is a bare Exception, not ValueError.

Source

Thrown at physics/speeds_of_gas_molecules.py:78

    >>> avg_speed_of_molecule(273, 0.028) # nitrogen at 273 K
    454.3488755062257
    >>> avg_speed_of_molecule(300, 0.032) # oxygen at 300 K
    445.5257273433045
    >>> avg_speed_of_molecule(-273, 0.028) # invalid temperature
    Traceback (most recent call last):
        ...
    Exception: Absolute temperature cannot be less than 0 K
    >>> avg_speed_of_molecule(273, 0) # invalid molar mass
    Traceback (most recent call last):
        ...
    Exception: Molar mass should be greater than 0 kg/mol
    """

    if temperature < 0:
        raise Exception("Absolute temperature cannot be less than 0 K")
    if molar_mass <= 0:
        raise Exception("Molar mass should be greater than 0 kg/mol")
    return (8 * R * temperature / (pi * molar_mass)) ** 0.5


def mps_speed_of_molecule(temperature: float, molar_mass: float) -> float:
    """
    Takes the temperature (in K) and molar mass (in kg/mol) of a gas
    and returns the most probable speed of a molecule in the gas (in m/s).

    Examples:

    >>> mps_speed_of_molecule(273, 0.028) # nitrogen at 273 K
    402.65620702280023
    >>> mps_speed_of_molecule(300, 0.032) # oxygen at 300 K
    394.8368955535605
    >>> mps_speed_of_molecule(-273, 0.028) # invalid temperature
    Traceback (most recent call last):
        ...
    Exception: Absolute temperature cannot be less than 0 K

View on GitHub (pinned to f5988cc097)

Solutions

1. Pass molar mass in kg/mol - N2 is 0.028, O2 is 0.032, CO2 is 0.044; divide g/mol values by 1000.
2. Validate molar_mass > 0 before the call, especially when it comes from a species lookup that can return 0/None.
3. Because this is a bare Exception, catch Exception (and check the message) or pre-validate rather than expecting ValueError.
4. Guard gas-name-to-molar-mass mappings with a KeyError/None check so unknown species never reach this function as 0.

Example fix

# before
avg_speed_of_molecule(273, 0)  # missing lookup -> 0

# after
MOLAR_MASS_KG = {'N2': 0.028, 'O2': 0.032, 'CO2': 0.044}
m = MOLAR_MASS_KG[gas]  # KeyError surfaces the real problem
if m <= 0:
    raise ValueError(f'bad molar mass for {gas!r}')
avg_speed_of_molecule(273, m)

if molar_mass is None or molar_mass <= 0:
    raise ValueError(f'molar_mass must be > 0 kg/mol, got {molar_mass!r}')
v = avg_speed_of_molecule(temperature, molar_mass)

def is_valid_molar_mass_kg(m: float) -> bool:
    return isinstance(m, (int, float)) and 0 < m < 1.0  # kg/mol values are < 1

try:
    avg_speed_of_molecule(t, m)
except Exception as e:
    if 'Molar mass' in str(e):
        ...

When it happens

Trigger: Calling avg_speed_of_molecule(273, 0) or with a negative second argument, e.g. avg_speed_of_molecule(273, -0.028).

Common situations: Passing molar mass in g/mol (28) instead of kg/mol (0.028) usually gives a wrong but non-raising answer, while passing 0 happens when a lookup table misses the gas; using an atomic mass for a diatomic gas without multiplying by 2 and hitting an uninitialized 0 default; parsing failures that yield 0 for unknown species strings.

Related errors

• Absolute temperature cannot be less than 0 K
• Expected b_coeffs to have {self.order + 1} elements for {sel
• k must not be negative
• Input value must be a positive integer
• Donor concentration should be greater than intrinsic concent

AI-assisted analysis of TheAlgorithms/Python@f5988cc097 (2026-08-14). Data as JSON: /api/errors/a4c614376254fc69. Report an issue: GitHub.