# The atomic mass of potassium is 39.1. What is the mass of #6.02 * 10^23# atoms of potassium?

##### 1 Answer

#### Explanation:

The first thing to note here is that the **atomic mass**, **not**

The *relative atomic mass* of potassium,

#A_(rcolor(white)(a)"K") = (39.1 color(red)(cancel(color(black)("u"))))/(1color(red)(cancel(color(black)("u")))) = 39.1#

So remember, the **atomic mass** of an element is **not** a unitless quantity. Atomic masses are expressed in *unified atomic mass units*,

Relative atomic masses are unitless because they express the mass of an element **relative** to

Now, all you have to do here is remember that **one mole** of a given element is defined as **atoms** of that element **Avogadro's number**.

In your case, **atoms** of potassium will be equivalent to **one mole** of potassium. Here is where the cool part comes in.

The **atomic mass** of potassium is equal to

#m_a = "39.1 u"#

By definition, the unified atomic mass unit is equal to

#color(purple)(|bar(ul(color(white)(a/a)color(black)("1 u " = " 1 g mol"^(-1))color(white)(a/a)|)))#

This means that the **molar mass** of potassium, which tells you the mass of **one mole** of the element, will be equal to

#39.1 color(red)(cancel(color(black)("u"))) * "1 g mol"^(-1)/(1color(red)(cancel(color(black)("u")))) = "39.1 g mol"^(-1)#

Therefore, you can say that **one mole** of potassium has a mass of