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6.8 mole of O₂

Explanation:

Given expression:

       2H₂   +   O₂   →   2H₂O

Number of moles of H₂ = 13.6moles

Unknown:

Number of moles of O₂  = ?

Solution:

In the given problem, we are to find the number of moles of oxygen gas that will use up 13.6mole of hydrogen gas;

 From the reaction equation;

             2 mole of H₂ will completely react with 1 mole of O₂

         13.6 moles of H₂ will completely be used up by mole of  O₂

                                                                                        = 6.8 mole of O₂

2. No graduation marks appear above the 0 mL mark.

Explanation:

A buret is a long graduated tube often used to administrate measured volumes of liquid, it has an opened extreme with a closed extreme in which the pass of the liquid can be determined, they often start at 0 ml and go down depending on the graduation. That´s why there is no graduation marks above the 0.