This unit we'll be on unit conversions. This is a basic skill that you're gonna be using
over and over in general chemistry. The units that we'll be using in general chemistry are
the SI units the simple units that we'll be starting with are mass which is given in kilograms,
length meters, temperature in Kelvin, and time in seconds there are other SI units that
you'll be learning as we go through the course but this is a good set to start with. There
also some SI prefixes. These prefixes allow us to change simply between grams to kilograms
or milligrams. The ones you'll be using most often are kilo, milli, and micro. Kilo is
1000 or 10 to the third in scientific notation. Milli is 1/1000 or 10 to the minus third in
scientific notation and micro is 1/1000000 or 10 to the -6 in scientific notation. This
allows us to convert easily between one unit and another. If we think about milligrams
to grams to kilograms. It takes 1000 mg to equal 1 g. It takes 1000 g to equal 1 kg this
allows us to do a simple problem. Let's convert 525 mg to kilograms. Now if we think about
the mathematics that's going into this we need to think about the fact that 1 kg is
equal to 1000 g. If we divided 1 kg by 1000 g because they are equal that quantity would
be one. We also should remember that if we take any quantity and multiply it by one we'll
get the same value. This allows us to convert easily using dimensional analysis and the
simple representation 1 kg equals 1000 g and 1 g equals 1000 mg. We want to start with
the 525 mg and convert that into kilograms. Now I'm going to use the simple dimensional
analysis graph here. I'll show you in a second what this means with just proportions. If
we start with 525 mg we have milligrams on the top so therefore we want milligrams on
the bottom. We have a simple way to go from milligrams to grams and so therefore we can
put grams on top. We can also then see that we can put grams on bottom and kilograms on
top. If we have 1000 mg is equal to 1 g and 1000 g is equal to 1 kg putting those all
in we check for our units we see that milligrams will cancel we see that grams can cancel and
we just do the mathematics to come up with our value 0.000525 kg. We can represent this
dimensional analysis chart also in just simple multiplication. So therefore we take 525 mg
times 1 g over 1000 mg times 1 kg over 1000 g and we get of course the same value. It's
simple to convert between the same system values using these SI prefixes. What you'll
also have to do is convert between unit systems. The conversion between unit systems is a little
bit more complex and we will need some conversion factors the reason that we need to convert
from SI units to another system is that also science uses SI units for the most part there
are other disciplines that do not follow the SI unit system. In fact if we think about
the United States the United States does not use the standard international units. If we
think about a simple question, how many miles would you run once you completed a 5K race.
Remember that a 5K race is actually 5 km so what we're gonna need to do is convert from
5 km to miles so therefore we're gonna need a conversion factor. Conversion factor 1 mile
is equal to 1.6093 km we can also say that one kilometer is equal to 0.62137 miles. If
we set up the same system as before we're gonna start with 5 km and we want to end up
with miles. We want the same unit on the bottom so therefore we're gonna have kilometers on
the bottom and miles on the top we can use any of these systems here. I'm going to choose
the first one which had 1.6093 km is equal to 1 mile. We do the mathematics and we see
that a 5 km race is 3.1069. Unit conversions are simple to do but it's very important that
you practice you need to be very competent at doing these calculations because they're
essential to many of the more advanced calculations you're gonna be doing later in the course.
Practice these become reliable and confident that you know how to do unit conversions.