The instructor likely marked the student's answer as wrong because of a significant issue with significant figures. When performing calculations, it's essential to consider the number of significant figures in the given data and carry them through the calculation to the final result.
In this case, if the initial data provided to the student had fewer significant figures than the student's answer of 90.32, the student's answer should be rounded to match the precision of the original data. If the original data had fewer significant figures, the answer should be rounded accordingly.
For example, if the initial data provided was 50.0 grams of calcium carbonate (with three significant figures), the answer should be reported with three significant figures as well. Therefore, the correct answer would be 90.3 grams of calcium oxide, rounding the last digit to match the precision of the original data.
In scientific calculations, it's crucial to follow the rules of significant figures to ensure the accuracy and precision of results.