# Question #dcad3

##### 1 Answer

#### Explanation:

We're asked to find the **hydrogen ion concentration**,

To do this, we can use the following relationship that is worth knowing:

#ul("pH" = -log["H"^+]#

Which we can rearrange to solve for the hydrogen ion concentration:

#ul(["H"^+] = 10^(-"pH")#

We're given that the

#["H"^+] = 10^-7.90 = color(red)(ulbar(|stackrel(" ")(" "1.3xx10^-8color(white)(l)"mol"*"L"^-1" ")|)#

Becuase the **decimal places**, the concentration is expressed with two **significant figures**.

The reason behind this comes from the fact that only the decimal places are important when calculating the logarithm. Basically, if the *which contains two significant figures also*), then the

#"pH" = -log(1.3xx10^-13color(white)(l)"mol"*"L"^-1) = 13#

if we only used two significant figures. This value isn't very precise, so we just use two *decimal places*:

#"pH" = 12.87#