# Tutor profile: Tressa S.

## Questions

### Subject: Chemistry

When methane reacts with oxygen a combustion reaction occurs resulting the carbon dioxide and water vapor being produced. If 5 grams of methane and 5 grams of oxygen react how many grams of carbon dioxide would be produced?

This is a stoichiometry problem where you must determine the limiting reactant before you can solve it. I am going to break this down in to steps. 1) Balance the chemical equation. You can't do anything until you have a balanced equation. For this reaction you would have the following: CH4 + 2O2 --> CO2 + 2H2O. Methane and carbon dioxide would have coefficients of 1 while oxygen and water would have coefficients of 2. 2) Determine your limiting reactant. There are multiple ways you can do this but I'm going to use the method of converting one reactant into another. I will be converting oxygen into methane to see if I have enough methane to use all 5 grams of oxygen. First I will convert 5 grams of oxygen into moles using the molar mass (32.0 g/ mol). We use 32 instead of 16 because oxygen is diatomic. I will then convert moles of oxygen to moles of methane using the ratio (1 mol CH4 / 2 mol O2). This ratio comes from the coefficients in the balanced equation. Finally I will convert to the mass of methane using the molar mass of methane (16.05 g/mol). The molar mass is determined by adding the masses of all the elements in methane together. This will give me the answer 1.25 g methane, meaning if I wanted to use all 5 grams of oxygen I would need 1.25 grams of methane. The problem states that i have 5 grams of methane available therefore I have more than enough methane and oxygen is my limiting reactant. 3) Determine amount of carbon dioxide produced using the limiting reactant. I'm going to start with 5 grams of oxygen because that is my limiting reactant. I convert 5 grams of oxygen to moles using the molar mass of oxygen (32.0 g/mol). I will then convert from moles of oxygen to moles of carbon dioxide using the mole ratio (2 mol O2 / 1 mol CO2) from balanced equation. Finally I will convert from moles of carbon dioxide to grams of using the molar mass of carbon dioxide (44.01 g/ mol). The final answer is 3.44 grams of carbon dioxide is produced.

### Subject: Basic Chemistry

Which neutral element on the periodic table would have 15 protons and 15 electrons? How many neutrons would it have if the mass number is 31?

To find the correct element on the periodic table you must first look at the atomic number, the whole number found on top of each element on the periodic table. This represents the number of protons in an atom as well as the number of electrons when the atom is neutral. Elements on the periodic table are listed in numerical order so you count or look for number 15 which is the element Phosphorus, P. To find the number of neutrons you would subtract the number of protons from the mass number that was provided. You can use the following equation: mass number = neutrons + protons therefore, neutrons = mass number - protons Plug in the numbers we have available: 31 - 15 = 16 so there are 16 neutrons present in this atom of Phosphorus.

### Subject: Physics

What is the kinetic energy of an empty cart that travels 50 centimeters in 0.35 seconds? The cart has a mass of 53 kg.

For the problem above you must use the equation for kinetic energy: KE = 1/2 mv^2 where m represents mass and v represents speed. You must be sure that you use the proper units for this problem so mass must be in kilograms and speed must be in meters per second. We are given mass in this problem in kilograms so no conversions are necessary. To find the speed of the cart you must take the distance traveled over time. This would be 50 cm / 0.35 seconds, but as I previously stated the units for speed must be in m/s so the distance must be converted from centimeters to meters. To do this you must divide the distance by a value of 100 since 100 cm = 1 m. This will now give you a distance of 0.5 m. To find speed you would then use 0.5 m/0.35 s = 1.43 m/s. If you must have proper significant figures the answer is 1 m/s but we will use 1.43 m/s for a more precise answer. Now that you have speed you can solve for the kinetic energy by plugging in all the correct values: 1/2 (53 kg)(1.43m/s)^2 = 54.2 kg m^2/s^2 which translates into units of Joules so the final answer is 54.2 J.

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