In order to find the mass percent of a certain atom in a particular molecule, you must know the mass the atom contributes to the molecule and the total mass of the molecule. In this case we calculate the mass percent using the equation

Calculating the mass ofin the molecule is done by multiplying the atomic mass of aatom by the number of atoms of that element present in the molecule. There are sixatoms in the presented molecule. We can tell this becausehas a subscript of 2 and is contained in a set of parentheses that has a subscript of 3. This tells us that there are three sets of twoatoms in each of the molecules. The atomic mass of aatom is given as 52.00 amu, so we can multiply:

Now we need to calculate the total atomic mass of the molecule. We can do this by identifying the atomic mass of each element in the molecule, multiplying it by the number of that type of atom in the molecule, and summing the results together.

We then plug in the values into equation to find the mass percent.

First of all, the unit degrees Celsius should be converted to Kelvins by adding 273. This means that the temperature increases from 273 K to 323 K.

Next, the combined gas law can be used.

Plugging in the known values gives

and solving for the volume gives

Rearranging shows this is clearly equivalent to the correct answer,.

By Graham's Law,. The molar mass ofis 2 g/mol and the molar mass ofis 18 g/mol. Thus,

Recall thatThus, the volume of the solution increased from 30 mL to 3000 mL, which is a factor of 100. Thus, the new concentration must be 100 times less than the original concentration, and.

首先,注意热量的反应的产物. This means that the reaction releases heat and is exothermic, so the enthalpy change must be negative. Next, consider that 2 moles of sodium would react to produce 640 kJ. Thus, 1.5 moles of sodium would react to produce

.

Hence, the enthalpy change is.

The student who has the accurate determination is that the student's whose average is closest to that of the "known" value of the crucible. Not be confused with precision, accuracy is how close the measured value is to the actual value, while precision is how close the measured values are to each other.

To answer this question, you need to calculate the averages of each students' weighings by summing them and dividing by five, since five measurements were taken.

Student 2's data averages out to 43.36, which rounds to 43.4. This average has the smallest difference from that of the "known" value for the crucible, 43.4273 g. Student 2's data is the correct answer.