Video Transcript
Acetylene, C2H2, is used in welding torches. The combustion of acetylene is described by the balanced chemical equation. 2C2H2 gas plus 5O2 gas react to form 4CO2 gas plus 2H2O gas. If 8.5 moles of acetylene is burned, how many moles of oxygen gas must have been consumed in a complete reaction? Give your answer to two decimal places.
Let’s begin by examining the provided balanced chemical equation. We are told that the equation represents a combustion reaction, which is a reaction with oxygen gas that produces energy in the form of heat and light. There are two reactants in this reaction, the fuel acetylene or C2H2 and oxygen gas or O2. During the combustion reaction, acetylene is burned in oxygen to form two different products. One product is carbon dioxide gas, and the other product is water vapor.
During any reaction, the reactants are the species that are consumed or used up. And the products are the new species formed. We know that 8.5 moles of the reactant acetylene gets used up during the reaction. However, we are not given the amounts of any other species in this reaction. We are asked to determine how many moles of oxygen gas are consumed when 8.5 moles of acetylene reacts. To help us solve this problem, we need to identify the relationship between acetylene and oxygen in the provided balanced equation.
In the given chemical equation, stoichiometric coefficients are used in front of each species involved in the reaction. A stoichiometric coefficient tells the number of moles of each species in a complete reaction. In this question, we are most concerned with the coefficients in front of C2H2 and O2. The coefficient of two in front of C2H2 indicates that there are two moles of acetylene, and the coefficient of five in front of O2 indicates that there are five moles of oxygen gas. This means that five moles of O2 are needed to completely react with two moles of C2H2.
When we write the amount in moles of two substances in a balanced equation as a ratio, it is called the molar ratio. Now we need to use the molar ratio to determine how many moles of O2 will be required to react with 8.5 moles of C2H2. We can use dimensional analysis to complete the calculation. Our plan is to multiply the given amount of moles of C2H2 from the problem, which is 8.5 moles, by the fraction form of the molar ratio. In the fraction form of the molar ratio, we’ll want to put the desired units in the numerator and the undesired units in the denominator.
We want to end up with the units of moles of O2 in our answer. So we need to put five moles of O2 in the numerator and two moles of C2H2 in the denominator. To perform the calculation, we’ll need to multiply 8.5 by five and divide the product by two. This gives us an answer of 21.25 moles of O2. The undesired units of moles of C2H2 were cancelled. We did not cross out the units of moles of O2. These were the desired units of our answer. Finally, we were asked to give our answer to two decimal places. The calculated answer is already given to two places past the decimal, so no rounding is needed. To burn 8.5 moles of acetylene gas, 21.25 moles of oxygen gas are required.
