During the past week and a half we have explored the idea of counting atoms by studying topics including relative mass, Avogadro’s number (the mole), and various mole calculations. Through the counting particles unit we expanded upon prior knowledge of the periodic table to eventually be able to count the number of atoms or molecules in a substance. I learned a lot more about how useful the periodic chart really is and started to realize just how tiny atoms really are.
The opening activity for unit five was a lab activity in which we determined the relative mass of different kinds of food samples and learned to count by massing. This was the Popcorn, Beans, and Rice Counters Lab where we weighed a dish of 12 kernels of popcorn, grains of rice, and beans and then wrote “For every” statements like; “For every 1 dozen kernels of popcorn there is 1.87 grams of mass,” for each one. From here we used the mass numbers to solve story problems that involved counting by weighing. We then used fractions to relate the three masses to the mass of the rice i.e. Rice: 1/1 Popcorn: 6.93/1 Beans 21.04/1, to introduce us to relative mass. The majority of these relative mass calculations were familiar to me from using ratios in other mass and science classes but until now I didn’t realize how useful ratios and proportions really are in chemistry.
This concept brought us to a worksheet where we examined the relative mass of oxygen to be 16:1 since oxygen is 16 times as dense and hydrogen. The next way we found relative mass on this worksheet was through percent composition of compounds. Here we looked at how elements like Nitrogen and Iron combine with oxygen in a one to one ratio and calculated the mass of these elements relative to that of Hydrogen, as John Dalton did back in the early 1800’s. After this we learned that all atomic masses on the periodic chart are relative masses based on Carbon 12=12.00. This 12.00 is the relative atomic mass measured in imaginary units called a.m.u.’s. Since a.m.u.’s are not measureable we needed to convert them to grams using Avogadro’s number; leading us into our work with moles.
Avogadro’s number is the massive 6.02×1023 and is one of the most important numbers in chemistry. Multiplying the 12.00 a.m.u.’s that represent the mass of one atom of carbon by 6.02×1023 shows that 6.02×1023atoms are equal to 12 grams of carbon. The more commonly used name for Avogadro’s number is a Mole. One mole represents 6.02×1023 of anything just 1 dozen represents 12 of anything. The mass of one mole of atoms/molecules is expressed in grams and is referred to as the molar mass. Through many examples of what 1 mole of various objects like marshmallows, rice, and basketballs, I learned just how huge the number 6.02×1023 really is, and just how small atoms and molecules are.
In our next whiteboard activities we learned how to use this number in mole calculations in order to eventually be able to count atoms by weighing. We started with “For every” statements relating the number of moles, number of atoms/molecules, and mass of the element copper. The first was; “For every 1 mole of Cu there are 6.02×1023 atoms of Cu.”
