# The density of water is 1.00 g/mL at 4°C. How many water molecules are present in 2.46 mL of water at this temperature?

##### 1 Answer

#### Explanation:

The idea here is that you need to use the density of water at *mass*, then use water's molar mass to find how many *moles* of water you get in this sample.

So, density is defined as mass per unit of volume. In this case, a density of **every milliliter** of water has a mass of

This means that

#2.46color(red)(cancel(color(black)("mL"))) * "1.00 g"/(1color(red)(cancel(color(black)("mL")))) = "2.46 g"#

Now, water has *molar mass* of **every mole** of water has mass of

#2.46color(red)(cancel(color(black)("g"))) * "1 mole water"/(18.015color(red)(cancel(color(black)("g")))) = "0.1366 moles water"#

Finally, the relationship between *number of moles* and *number of molecules* is given by **Avogadro's number**, which tells you that **every mole** of a substance contains exactly

In this case, you will have

#0.1366color(red)(cancel(color(black)("moles"))) * (6.022 * 10^(23)"molecules")/(1color(red)(cancel(color(black)("mole")))) = color(green)(8.23 * 10^(22)"molecules")#

So,