Megan T. answered • 11/02/20
Efficacious tutor teaching sciences, SAT preparation, and math.
a) They already gave you your actual yield of iron in grams, so a grams to moles conversion of iron will give you the actual yield. To convert from grams to moles, one must divide the given amount of grams by the molar mass of iron to convert to moles. The molar mass of iron can be found on the periodic table under the element symbol in the center. If the question had not stated "excess carbon," you would have to identify the limiting reactant prior to completing anything else. If it was a naturally occurring diatomic element, we would have to double the molar mass. However, iron is not diatomic, so we will use 55.845 g/mol as the molar mass.
Therefore, the initial setup would look like this:
18.1 g Fe l moles Fe l moles Fe
l grams Fe l
Once the molar mass has been inserted and the calculations are complete, this is how it should look:
18.1 g Fe l 1 mol Fe l .3241 moles Fe
l 55.845 g Fe l
Remember that when utilizing dimensional analysis, your units must cross each other out and only units on top and bottom can cross out. If you remember this, you will be able to identify whether to multiply or divide by the molar mass based on the starting unit. The given amount and unit is always placed in the top left position and the desired unit is placed in the top right position. The conversion factor(s) is(are) always placed in the center slots.
Do not forget to consider significant figures. There are three significant figures given in the question, so the answer would be .324 moles Fe
b) The theoretical yield is what you will get when you utilize the starting amount of Iron and the molar ratio of Iron (III) Oxide as compared to Iron (given by the coefficients of these compounds in the reaction) in order to ascertain the hypothetical amount if there were no confounding variables and no possibility of experimental error.
.318 moles Fe2 O3 l 2 moles Fe l .636 moles Fe
l 1 mole Fe2 O3 l
Clearly, there was a substantial amount of the sample lost during the experiment when the theoretical yield is .636 moles Fe (three significant figures) and the actual yield is only .324 moles Fe. This question could have required a conversion from grams to moles prior to completing the molar ratio if it gave the initial amount in grams, requiring the division of the amount by its molar mass. Additionally, it could have required a moles to gram conversion if it had requested the theoretical yield in grams rather than moles. If we are only converting between reactants and products rather than units, it will only require the use of the molar ratio to identify the solution. Make sure the reaction is balanced before using the coefficients in the molar ratios. Sometimes the question will require you to balance the equation yourself.
