Question

The chemical analysis of the major components of a seawater sample is given below (solutes in terms of mg/kg solution)

Cl-	18980	Ca2+	400
Na+	10561	K+	??
Mg2+	??	Br-	65
SO42-	??	HCO3-	28

3.a. Calculate the concentrations (units of moles per kg of solution) of all of the species listed.

3.b. Calculate the salinity of this sample.

3.c. Calculate its ionic strength

Answer 1

Salinity is the total of all non-carbonate salts dissolved in water, usually expressed in parts per thousand (1 ppt = 1000 mg/L). Unlike chloride (Cl–) concentration, you can think of salinity as a measure of the total salt concentration, comprised mostly of Na+ and Cl– ions. Even though there are smaller quantities of other ions in seawater (e.g., K+, Mg2+, or SO4 2–), sodium and chloride ions represent about 91% of all seawater ions. Salinity is an important measurement in seawater or in estuaries where freshwater from rivers and streams mixes with salty ocean water. The salinity level in seawater is fairly constant, at about 35 ppt (35,000 mg/L), while brackish estuaries may have salinity levels between 1 and 10 ppt. Since most anions in seawater or brackish water are chloride ions, salinity can be determined from chloride concentration.

The following formula is used to calculate salinity of water in terms of chlorides ions concentration,

salinity (ppt) = 0.0018066 ✕ Cl– (mg/L) =

                     = 0.0018066 × 18980

                     = 34.28 ppt = 34,280 mg/L

Answer 3C )

I = 1/2 Σ mi . Zi2

Z= charge of i th ion

m= concentration of i th ion

I_SW (Ionic strength of salt water ) = 1/2 (m_Na x 1² + m_Mg x 2² + m_Ca x 2² + m_K x 1² + m_Cl x 1² + m_SO4 x 2² + m_HCO3 x 1²)

I_SW (Ionic strength of salt water ) = ½ (10561 ×1) + (0 ×4) + (400× 1) + ( 0×1) + (18980 × 1) + ( 0 × 4) + ( 28 × 1)

I_SW (Ionic strength of salt water ) = 29969 mg/Kg

Since concentration of Mg, K and SO4 is not mention in the table we couldn’t use it in the above calculation of ionic strength.

More information is require to calculate all species concentration in question 3a