# Tutor profile: Crystal W.

## Questions

### Subject: Basic Math

What is PEMDAS or order of operations?

PEMDAS or order of operations are rules determining the order in which mathematical operations such as addition, subtraction, multiplication, and division should be performed when multiple operations are present in one math problem. PEMDAS stands for parentheses, exponents, multiplication, division, addition, and subtraction. This mnemonic device tells us what order mathematical operations should be performed in. For example, if we were presented with the following problem.... (2x4-5)/6 ....following order of operations, the first operation we would perform is the multiplication operation in the parentheses.

### Subject: Chemistry

Explain how to use A-values in order to calculate the most stable orientation of substituents in a cyclohexane ring (conformational analysis).

A-values are used to predict the most stable conformation of cyclohexane rings in the chair conformation. Substituents on the ring can be in either the axial (up and down) or equatorial (left and right) position. The most stable conformation results when crowding is minimized between the axial substituents on the ring. Bulkier groups have larger A-values and therefore will cause more crowding in the axial position. The strain caused by these groups will lead to a less stable molecule. To determine the stability of a conformation, find the sum of the A-values of the substituents in the axial positions on the cyclohexane ring. The greater the sum of the A-values, the less stable the conformation is. The cyclohexane ring will prefer the conformation in which the sum of the A-values are minimized. Therefore, one can determine the most stable conformation by finding the conformation with the smallest sum of A-values. Overall, by minimizing the sum of the A-values of the axial substituents in a cyclohexane chair conformation, we are able to determine the most stable conformation.

### Subject: Basic Chemistry

Why is balancing chemical reactions important for stoichiometric calculations?

It is important to balance chemical reactions because it allows us to obtain the correct stoichiometric coefficients in front of each chemical species in the reaction. These stoichiometric coefficients allow us to relate the mols of one species in the reaction to another. From these coefficients, we are able to obtain the correct mole ratios (conversion factors that relate the amounts in moles of any two substances in a chemical reaction). From this, we can perform stoichiometric calculations. For example, from the balanced chemical reaction H2+O2->2H2O, we are able to write the conversion factor 1 mol H2/2 mols H2O. From the balanced reaction, we are able to see that 1 mol of hydrogen gas reacts to give us 2 mols of water.

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