C2H2 will be left when the burning is complete.
The equation of the reaction is; 2C2H2 + 5O2 --> 4CO2 + 2H2O
The number of moles of C2H2 reacting is = 1.20 g/26 g/mol = 0.046 moles
The number of moles of O2 is = 3.50 g/32 g/mol = 0.109 moles
Since;
2 mole of C2H2 reacts with 5 moles of O2
x moles of C2H2 reacts with 0.109 moles of O2
x = 2 mole × 0.109 moles/5 moles
x = 0.044 moles of C2H2.
It then follows that C2H2 is the reactant in excess so C2H2 will be left when the burning is complete.
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