Answer:
11 molecules of CH4.
23 atoms of C is the leftover.
Explanation:
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In this case, for the formation of methane:

We can see there is an excess of carbon based on their stoichiometry, because the needed amount of hydrogen gas molecules would be:

Thus, the formed molecules of methane are computed below:

In such a way, the leftover of carbon atoms are:

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O valence electron number is the answer
Answer:
Percentage error = 1.88 %
Solution:
Data Given:
Mass of Sample = 20.46 g
Volume of Sample = 43.0 mL - 40.0 mL = 3.0 mL
Formula Used:
Density = Mass / Volume
Putting values,
Density = 20.46 g / 3.0 mL
Density = 6.82 g.mL⁻¹
Percentage Error:
Experimental Value = 6.82 g.mL⁻¹
Accepted Value = 6.95 g.mL⁻¹
= 6.82 g.mL⁻¹ / 6.95 g.mL⁻¹ × 100 = 98.12 %
Percentage Error = 100 % - 98.12 %
Percentage error = 1.88 %