# Question f517b

Jul 30, 2017

$3$ ${\text{g O}}_{2}$ (one significant figure)

#### Explanation:

We're asked to find the mass of ${\text{O}}_{2}$ required to completely combust $2$ $\text{g C}$.

To do this, we'll first write the balanced chemical equation for this reaction:

${\text{C"(s) + "O"_2(g) rarr "CO}}_{2} \left(g\right)$

(note: if this were incomplete combustion, carbon monoxide would be produced instead)

Let's use the molar mass of carbon ($12.011$ $\text{g/mol}$) to find the number of moles that burn:

2cancel("g C")((1color(white)(l)"mol C")/(12.011cancel("g C"))) = color(red)(0.1665 color(red)("mol C"

Now, we can use the coefficients of the chemical equation to find the relative number of moles of ${\text{O}}_{2}$ that burn:

color(red)(0.1665)cancel(color(red)("mol C"))((1color(white)(l)"mol O"_2)/(1cancel("mol C"))) = color(green)(0.1665 color(green)("mol O"_2

Lastly, we'll use the molar mass of oxygen gas ($31.999$ $\text{g/mol}$) to calculate the number of grams of oxygen:

color(green)(0.1665)cancel(color(green)("mol O"_2))((15.999color(white)(l)"g O"_2)/(1cancel("mol O"_2))) = color(blue)(ul(3color(white)(l)"g O"_2

which I suppose will leave at one significant figure (the more precise value is color(blue)(2.66 color(blue)("g O"_2#).