# Tutor profile: Kurt C.

## Questions

### Subject: Chemistry

What is the percent yield for a reaction of 4.00 grams of oxygen gas and an excess of hydrogen gas to form 4.25 grams of water.

First, write the balanced equation for the reaction. O2(g) + 2H2(g) 2H2O(g) Then, find the moles of the limiting reagent. In this case, oxygen is the limiting reagent because an excess of hydrogen is used. To find the moles of oxygen, divide the grams of oxygen gas by its molar mass. Oxygen’s molar mass can be found on the periodic table. Oxygen’s molar mass is 15.999 grams per mole. Because oxygen gas is diatomic, you must multiply the molar mass of oxygen by 2. This means the molar mass for oxygen gas is 31.998. If you divide the 4.00 grams of oxygen gas by 31.998 grams per mole, you get 0.125 moles of oxygen. Then, you use the ratio of the limiting reagent to the product to find the theoretical yield. Theoretical yield is the yield you would get if of the limiting reagent reacts to form product. The ratio can be found in the balanced equation. For every mole of oxygen gas reacted, 2 moles of water is formed. Therefore, multiply the moles of oxygen gas times 2 to find the moles of water you would form. 0.125 moles times 2 equals 0.250 moles. 0.250 moles of oxygen is the theoretical yield. You want to convert this to grams of water in order to find the percent yield. To convert 0.250 moles of water into grams of water, multiply the moles of water times water’s molar mass. The molar mass of water is the molar mass of oxygen plus 2 times the molar mass of hydrogen. These can be found on the periodic table. The molar mass of water is 18.02 grams per mole. 0.250 moles times 18.02 grams per mole is 4.51 grams. 4.51 grams is your theoretical yield of water. Now you can find percent yield by dividing the actual yield, 4.25 grams, by the theoretical yield, 4.51 grams, and then multiplying by 100. The percent yield is 94.2 %.

### Subject: Basic Chemistry

Which state of matter has a defined volume, but not a defined shape? A) Solid B) Liquid C) Gas

The answer is B) Liquid. Liquids have no defined shape. Instead, liquids take the shape of their container much like gases do. However, liquids have a defined volume because they are not compressible. Gases can be compressed thus changing their volume. Solids have both a defined shape and volume.

### Subject: Algebra

Find the slope of line that passes between the points (2,4) and (4,8).

The slope of a line is rise over run. This can be expressed in the equation (y2-y1) / (x2-x1). If we plug the coordinates into this equation, we get (8-4) / (4-2). (8-4) = 4 and (4-2) = 2 4/2= 2 Therefore, the slope is 2.

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