Which substance has a molecular mass of 44 u?

The correct option is Carbon dioxide.

To find which substance has a molecular mass of 44 u, we need to calculate the molecular mass of each compound given in the options by summing the atomic masses of all the constituent atoms.

Let us recall the standard atomic masses of the involved elements:

• Atomic mass of Carbon (C) = 12 u
• Atomic mass of Oxygen (O) = 16 u
• Atomic mass of Hydrogen (H) = 1 u
• Atomic mass of Nitrogen (N) = 14 u

Now, let us calculate the molecular mass for each option step-by-step:

1. Carbon monoxide (CO):
Molecular mass of CO = (1 × Atomic mass of C) + (1 × Atomic mass of O)

$\text{Molecular mass}=12+16=28\text{u}$

2. Carbon dioxide (CO₂):
Molecular mass of CO₂ = (1 × Atomic mass of C) + (2 × Atomic mass of O)

$\text{Molecular mass}=12+(2\times 16)=12+32=44\text{u}$

3. Methane (CH₄):
Molecular mass of CH₄ = (1 × Atomic mass of C) + (4 × Atomic mass of H)

$\text{Molecular mass}=12+(4\times 1)=12+4=16\text{u}$

4. Ammonia (NH₃):
Molecular mass of NH₃ = (1 × Atomic mass of N) + (3 × Atomic mass of H)

$\text{Molecular mass}=14+(3\times 1)=14+3=17\text{u}$

Therefore, Carbon dioxide (CO₂) has a molecular mass of exactly 44 u.