Unknown Mass in Dalton's Law of Partial Pressures
Assuming that the total pressure $p_\text{total}=2068.44\ \mathrm{kPa}$ is equal to the sum of the partial pressures of the individual gases in accordance with Dalton’s law, the partial pressure of air $p_\text{air}$ can be calculated as follows:

$$\begin{align}
p_\text{total}&amp;=p_\text{air}+p_{\ce{CO2}}\\[6pt]
p_\text{air}&amp;=p_\text{total}-p_{\ce{CO2}}\\[6pt]
&amp;=2068.44\ \mathrm{kPa}-344.74\ \mathrm{kPa}\\[6pt]
&amp;=1723.70\ \mathrm{kPa}
\end{align}$$

Since molar mass $M$ is defined as

$$M=\frac mn$$

where $m$ is mass and $n$ is amount of substance, the ideal gas law 

$$p\cdot V = n\cdot R\cdot T$$

where $V$ is volume, $R$ is the molar gas constant, and $T$ is temperature, can be rewritten as

$$\begin{align}
p\cdot V&amp;=\frac{m\cdot R\cdot T}M\\[6pt]
\frac V{R\cdot T}&amp;=\frac m{M\cdot p}
\end{align}$$

This equation applies to air
$$\frac V{R\cdot T}=\frac {m_\text{air}}{M_\text{air}\cdot p_\text{air}}$$
where $m_\text{air}=0.454\ \mathrm{kg}$ is the given mass of air and $M_\text{air}=28.97\ \mathrm{g\ mol^{-1}}$ is the average molar mass of air, as well as to $\ce{CO2}$
$$\frac V{R\cdot T}=\frac {m_{\ce{CO2}}}{M_{\ce{CO2}}\cdot p_{\ce{CO2}}}$$
where $m_{\ce{CO2}}$ is the unknown mass of $\ce{CO2}$ and $M_{\ce{CO2}}=44.01\ \mathrm{g\ mol^{-1}}$ is the molar mass of $\ce{CO2}$.

Therefore,

$$\begin{align}
\frac {m_{\ce{CO2}}}{M_{\ce{CO2}}\cdot p_{\ce{CO2}}}&amp;=\frac {m_\text{air}}{M_\text{air}\cdot p_\text{air}}\\[6pt]
m_{\ce{CO2}}&amp;=\frac {M_{\ce{CO2}}\cdot p_{\ce{CO2}}\cdot m_\text{air}}{M_\text{air}\cdot p_\text{air}}\\[6pt]
&amp;=\frac {44.01\ \mathrm{g\ mol^{-1}}\times344.74\ \mathrm{kPa}\times0.454\ \mathrm{kg}}{28.97\ \mathrm{g\ mol^{-1}}\times1723.70\ \mathrm{kPa}}\\[6pt]
&amp;=0.138\ \mathrm{kg}
\end{align}$$

Note that the given value for the volume $V=85\ \mathrm l$ is not required for the calculation of $m_\text{air}$. Thus, the uncertainty of the volume $V=85\ \mathrm l$ (with only two significant digits) does not affect the uncertainty of the result.