Everything Science Grade 10

CHAPTER 1. SKILLS FOR SCIENCE 1.3

Changing the subject of a formula ESAL

Very often in science you will have to change the subject of a formula. We will look at two
examples. (Do not worry if you do not yet know what the terms and symbols mean, these
formulae will be covered later in the book.)

1 .Moles

The equation to calculate moles from molar mass is:n=Mm, wherenis the number

of moles,mis the mass andMis the molar mass. As it is written we can easily find

the number of moles of a substance. But what if we have the number of moles and

want to find the molar mass? We note that we can simply multiply both sides of the

equation by the molar mass and then divide both sides by the number of moles:

n = Mm

nM = m

M = mn

And if we wanted the mass we would use:m=nM.

2. Energy of a photon

The equation for the energy of a photon isE =hcλ, whereEis the energy,his

Planck’s constant,cis the speed of light andλis the wavelength. To getcwe can do

the following:

E = hλc

Eλ = hc

c = Eλh

Similarly we can find the wavelength we use:λ=hcE and to find Planck’s constant

we use:h=Eλc.

Rate, proportion and ratios ESAM

In science we often want to know how a quantity relates to another quantity or how some-
thing changes over a period of time. To do this we need to know about rate, proportion
and ratios.
Rate:

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