Laboratory Techniques and Analysis - AP Chemistry

First calculate the number of mols of each reactant:

The ratio of to is 1:2. So, given that we have 0.048945 mol , we only need 0.09789 mol . Since we have more than is needed, is in excess. How much is in excess?

Now convert mols of to grams:

The system is open, so the pressure can be assumed atmospheric, e.g. 1 atm.

Using the ideal gas law,

We need to convert to kelvin.

The density of water at room temperature is approximately 1 g/mL, so we have 25 g of water. Convert to mols:

Plugging everything in:

First we need to calculate the mass of the wire. We will do this by calculating its volume then multiplying by the density.

Next we need to find out how much energy the water gained from the metal wire.

The energy the water gained is equal to the energy the wire lost. This allows us to calculate the heat capacity for the metal:

We are given the pressure, temperature, and molar mass of the gas in the question stem and are asked to determine the density. To do this, we will need to make use of a manipulated form of the ideal gas equation.

To find density, we will need a way to relate the variables given in the question stem to other parameters, namely mass and volume. Since we already have the volume, , contained in the ideal gas equation, we just need a way to find the mass, . We can do this by noting that the number of moles, , can also be written as the mass divided by the molar mass, . Substituting this in gives us:

And rearranging, we obtain the density:

But we're not done yet! Remember, we also have to convert the units given for pressure and temperature in the question stem into their proper form. Since we need pressure in terms of atmospheres, and there are in of gas, we can find that:

Furthermore, we need to convert degrees Celsius into Kelvins.

Now that we have all the units in their correct form, we're finally ready to plug our values into the equation for density:

This is a typical dilution problem. The first step is to recognize that this is a dilution question. Whenever figuring out how to go from a higher concentrated solution to a lower concentrated solution, it is a dilution problem. The main equation to use here is:

or

or

When plugging in the numbers, make sure to assign the correct volumes and concentrations. Think of one side of the equation describing the 0.9% solution of sodium chloride and the other side describing the 0.45% solution of sodium chloride.

is the amount of the original solution needed to make the new 0.45% solution. However, we need to add water to the solution to dilute it. To find how much water is needed, subtract:

of water.

42,000,000 has two significant figures because of the rule that states that trailing zeroes are never significant unless there is a decimal point. For example, if the number given was "42,000,000." it would be eight significant figures.

The correct answer is . This is found by moving the decimal place to the left 7 times in order to get one digit to the left of the decimal, and the rest of the digits to the right. The answer just has two significant figures because that is the number of significant figures in the original number.

To convert into standard notation we must move the decimal two places to the right because of the exponent of "2" above the 10. Then we re-write the number with the decimal in the new space. All of the digits that were in the original scientific notation must be accounted for. Thus, our answer must have six significant figures.

The answer is 500 because our rules of significant figures state that when two numbers are multiplied our answer can only have as many significant figures as our number with the fewest significant figures. In this case 5.02 has three significant figures, but 100 only has 1 significant figure. Therefore our answer can only have 1 significant figure.

The answer is 13.18 because the rules for adding and subtracting significant figures state that the answer can only be precise up to the least precise digit from the numbers that were added/subtracted. In this case 2.12 is only precise to the hundredth place, and therefore our answer can only go to the hundredth place.

Remember:

Rearrange the equation to solve for volume:

Plug in the values given in the problem:

However, we must take into account significant figures. Because both our given values have 3 significant figures, our answer must have 3 significant figures.

Therefore after rounding the correct answer is:

To find the percent error, the formula is:

Therefore we plug in the values given:

Therefore, the correct answer including significant figures is

The formula for percent error is:

Plug in the given values and solve:

Therefore, after accounting for significant figures the correct answer is

Group A's values are precise because the data values are all close to each other, but it is not accurate because it none of them is near the accepted value.

Group C is precise because all of their values are close to one another. Their data is also accurate because their values are all near the accepted value. Therefore group C is both accurate and precise.

Remember the saying "King Henry Died by Drinking Chocolate Milk". This is referencing the order of metric prefixes: kilo, hecto, deca, deci, centi, milli. Or in their abbreviations: k, h, da, d, c, m.

You also need to know nano (n) which is .

The final one you need to know for this question is Mega (M) which is .

Therefore the correct answer is: n, m, k, M (nano, milli, kilo, Mega).

The critical point for water is at 218atm and 374 degrees Celcius, but on this graph it can be found at the end of the line between the gas and liquid portions of the water phase graph (you can see it end with a dot). This means we need to increase our pressure and decrease our temperature in order to reach the critical point from our initial conditions.

A first-order reaction can be expressed in different ways. One way is the following:

where:

= concentration of reactant at a given time,

= initial concentration of reactant at time

= rate constant of the reaction

= amount of time that has passed since the start of the reaction

By taking the natural logarithm of both sides, we can obtain an alternative form of the equation, expressed as:

This second equation is useful for graphing, because it fits the general equation of . In this case, represents the -axis, represents the -axis, represents the -intercept, and represents the slope. Thus, the graph in the question stem is indeed indicative of a first-order reaction.

In the case of a zeroth-order reaction, the equation is:

Therefore, for a zeroth-order reaction, a straight line with a negative slope will only result from a graph of the concentration of a reactant versus time.

And lastly, in the case of a second-order reaction, the equation is:

Hence, a graph of the inverse concentration of a reactant versus time will yield a straight line with a positive slope, and this is indicative of a second-order reaction.

The number of significant figures in the final answer will ultimately be determined by the term in the equation that has the fewest significant figures. Let us examine the components of the expression we are given.

Since the least precise term in the expression has only one significant figure, our final answer will have only one significant figure.

When dividing two numbers, the answer can only have as many significant figures as the number with the fewest significant figures. The mass has 6 significant figures while the volume only has 2, therefore the answer can only have 2. Don't forget to round!