Gases such as carbon dioxide diffuse much more slowly in water than in air. Plants that are fully submerged have greater difficulty obtaining the carbon dioxide they need. To help ameliorate this problem, underwater leaves lack a waxy coating because carbon dioxide is easier to absorb without this layer.
The density of CO2 getting from experiment is 0.1/0.056 = 1.79 g/L. The percent error of this is (1.96 -1.79)/1.96*100%=8.67%. So the approximate percent error is 8.67%.