Which of these measurements is expressed as three significant figures?
C. 4.20 g
A buffer solution contains an equivalent amount of acid and base. The pH of the solution with an acid dissociation constant (pKa) value of 3.75 is 3.82.
<h3>What is pH?</h3>
The amount of hydrogen or the proton ion in the solution is expressed by the pH. It is given by the sum of pKa and the log of the concentration of acid and bases.
Given,
Concentration of salt [HCOO⁻] = 0.24 M
Concentration of acid [HCOOH] = 0.20 M
The acid dissociation constant (pKa) = 3.75
pH is calculated from the Hendersons equation as,
pH = pKa + log [salt] ÷ [acid]
pH = 3.75 + log [0.24] ÷ [0.20]
= 3.75 + log (1.2)
= 3.75 + 0.079
= 3.82
Therefore, 3.82 is the pH of the buffer.
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The volume of copper : 3.24 ml
<h3>Further explanation
</h3>
Density is a quantity derived from the mass and volume
Density is the ratio of mass per unit volume
With the same mass, the volume of objects that have a high density will be smaller than objects with a smaller type of mass
The unit of density can be expressed in g/cm³ or kg/m³
Density formula:

ρ = density
, g/cm³ or kg/m³
m = mass
, g or kg
v = volume
, cm³ or m³
A common example is the water density of 1 gr / cm³
The density of copper : 8.96 gr/ml
mass of copper : 29 g
then the volume :

Balance the equation first:
2 KClO3 (s) ---> 2 KCl (s) + 3 O2 (g)
Moles of KClO3 = 110 / 122.5 = 0.89
Following the balanced chemical equation:
We can say moles of O2 produce =

x moles of KClO3
So, O2 = (3 / 2) x 0.89
= 1.34 moles
So, Volume at STP = nRT / P
T = <span>273.15 K
P = 1 atm
So, V = (1.34 x 0.0821 x 273.15) / 1 = 30.2 L</span>