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Brittany A.
High School Chemistry Teacher
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Inorganic Chemistry
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Question:

30.0 mL of a 0.2 M HC2H3O2 (Ka = 1.8 x 10^-5) solution is titrated with 0.10 M NaOH. Calculate the pH of the solution when 30 mL of NaOH is added.

Brittany A.
Answer:

Since the Ka value is small, we can tell that HC2H3O2 is a weak acid. Therefore we have to use our ICE tables to solve for the equilibrium concentrations. Writing out our neutralization reaction for acetic acid with sodium hydroxide, we get HC2H3O2 + OH- --> H2O + C2H3O2- using the given concentration as well as the given volume, we can determine mmoles of each reactant used in the experiment. (Ex. 30 mL HC2H3O2/0.2 M = 6 mmol) Completing the same steps for OH, we would determine 3 mmol. Since it is a weak acid, we know it does not completely dissociate in solution, leaving 3 mmol of our conjugate base left over. From the mmoles and the total amount of volume in the solution, we can then calculate concentration of each species. Plugging that into our Ka equation, using our Ka value, we get 1.8 x 10^-5 = ({H+] x [0.05])/[0.05] = 1.8 x 10^-5. This becomes our concentration of H+ ions in solution. We can now plug that into pH = -log[H+} to get our pH of 4.74.

Chemistry
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Question:

In an experiment, sodium chloride reacts with magnesium carbonate in a double-displacement reaction. How much sodium chloride would need to be used in order to produce 15.0 g of magnesium chloride?

Brittany A.
Answer:

First, we need to write the balanced reaction. 2 NaCl + MgCO3 --> Na2CO3 + MgCl2 Second, start with what you are given. 15.0 g MgCl2 Third, we know that if we must get from one compound to a completely different compound that we must use our mole ratios (coefficients of the balanced equation). 1 mol MgCl2/2 mol NaCl Fourth, we set up our stoichiometry problem. 15.0 g MgCl2 x (1 mol MgCl2/95.1 g MgCl2) x (2 mol NaCl/1 mol MgCl2) x (58.5 g NaCl/1 mol NaCl) = 18.45 g NaCl needed to produce 15.0 g of MgCl2.

Basic Chemistry
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Question:

Give a molecular explanation for the relationship between pressure and volume.

Brittany A.
Answer:

Pressure and volume have an indirect relationship. This means as one increases the other decreases and vice-versa. Pressure is based on the number of collisions particles have not only with each other but with the container walls. As volume decreases, the number of collisions occur between the walls of the container and the particles themselves, thus increasing the overall pressure of the system.

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