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Mr Johnsonon 3 December 2012
Transcript of Percent Composition
compound is a RELATIVE MEASURE
of the mass of each different element present in the compound. The FORMULA of a compound shows its composition in terms of what elements it contains. It is often useful to know the composition of a compound in terms of the MASSES of its elements. For example, magnesium oxide decomposes to its elements magnesium and oxygen. 2MgO + heat --> 2Mg + O 2 The mass of magnesium obtained as product can be calculated from the mass percent of Mg in MgO. The composition by mass of a substance can be determined EXPERIMENTALLY by reacting a known quantity of the substance with a known quantity of another compound. The mass amounts can then be converted to PERCENTAGES in order to give us the percent composition. We can then use the atomic mass and molar mass to determine the correct formula. Let's look at the percent composition of water. Water is formed when hydrogen is allowed to react with oxygen in a process that gives off a large amount of heat. The results of an experiment showed that 2.50g of hydrogen COMPLETELY reacted to produce 22.50g of water. Since 2.50g of hydrogen combined with oxygen to yield 22.50g of water, we can calculate the mass of oxygen by subtraction. m = 2.50g H m = 22.50g H O 2 m = (22.50g - 2.50g) O To calculate the percent composition of each element in one molecule of water, we DIVIDE the mass of the element by the total mass of the molecule. %H = m H m H O 2 _________ x 100% %O = m O m H O 2 x 100% ___________ x 100% = 2.50 g 22.50g ___________ x 100% = 20.00 g 22.50g Sometimes we need to calculate the percent composition of a compound whose formula we know. If we already know the chemical formula, the calculations are pretty straightforward. First we calculate the
mass of all the atoms of each element. Then we calculate the total mass of all the elements in the compound. Finally, we divide the mass of each element by the total mass to get the percent composition. Na CO Consider sodium carbonate 2 3 m = 22.99u x 2 m = 12.01u x 1 m = 16.00u x 3 Na C O = 45.98u = 12.01u = 48.00u m = 105.99u tot Now that we have masses we can find the percent composition. 45.98u 105.99u = 43.38% 12.01u 105.99u = 11.33% %O = 48.00u 105.99u _________ = 45.29% The percentage composition of Na CO 2 3 43.38% Na 11.33% C 45.29% O HOMEWORK Percentage Composition Worksheet For example, when trying
to assess the purity of a compound. Calculate the
by mass of sodium sulphate,
Na SO . 2 4 MM = 22.99g/mol
MM = 32.06g/mol
MM = 16.00g/mol Na S O In an experiment, 8.40g of carbon reacts with oxygen and 30.80g of carbon dioxide is produced. Calculate the percent composition. The image was taken by Curiosity's Mast Camera on the 32nd Martian day, or sol, of operations on the surface (Sept. 7, 2012, PDT or Sept. 8, 2012, UTC). APXS can be seen in the middle of the picture. Portrait of APXS on Mars The red dots are where the Chemistry and Camera (ChemCam) instrument zapped it with its laser.
The circular black and white images were taken by ChemCam to look for the pits produced by the laser.
The purple circles indicate where the Alpha Particle X-ray Spectrometer trained its view. "This rock is a close match in chemical composition to an unusual but well-known type of igneous rock found in many volcanic provinces on Earth," said Edward Stolper of the California Institute of Technology in Pasadena, who is a Curiosity co-investigator. "With only one Martian rock of this type, it is difficult to know whether the same processes were involved, but it is a reasonable place to start thinking about its origin." The APXS carries a small alpha particle source. The alphas are emitted and bounce back from a science target into a detector in the APXS, along with some X-rays that are excited from the target in the process. The energy distribution of the alphas and X-rays measured by the detectors is analyzed to determine elemental composition. NASA's rover Curiosity touched a Martian rock with its robotic arm for the first time on Sept. 22, assessing what chemical elements are in the rock called "Jake Matijevic." Jake was the first rock analyzed by the rover's arm-mounted Alpha Particle X-Ray Spectrometer (APXS) instrument Grad student Nicholas Boyd (left) and Principal Investigator Ralf Gellert, both of the University of Guelph, Ontario, Canada, prepare for the installation of the Alpha Particle X-ray Spectrometer sensor head during testing at NASA's Jet Propulsion Laboratory. The calibration target for the Mars Hand Lens Imager (MAHLI) instrument includes color references, a metric bar graphic, a 1909 VDB Lincoln penny, and a stair-step pattern for depth calibration.