Question

After Na, K and Cl, seawater has a higher concentration of which element
A. Mg
B. Zn
C. Ca
D. Pb

Answer 1

Seawater or saltwater is water that comes from a sea or ocean. On average, seawater has a salinity of about 3.5% (35 g/l, 599 mM) in the world's oceans. It means that every kg of seawater has approximately 35 grams (1.2 oz) of dissolved salts (majorly sodium ($N{a^ + }$) and chloride ($C{l^ - }$) ions. Seawater is considered as much denser than both freshwater and pure water.

Complete answer:
Covering more than 70 percent of Earth’s surface, Seawater, water that makes up the oceans and seas. Seawater is a complex mixture comprising water, salts in the concentration of 96.5% and 2.5 % respectively, and smaller amounts of other substances, including dissolved inorganic and organic materials, particulates, and a few atmospheric gases.
The most abundant ions of seawater are the following: chloride ($C{l^ - }$), sodium ($N{a^ + }$), sulfate ($SO_4^{2 - }$), magnesium, calcium, and potassium (${K^ + }$). These ions make up about 99 percent of all sea salts, by weight. Because of the addition or removal of water locally through precipitation and evaporation, the amount of these salts in a volume of seawater varies.
After sodium, potassium & chlorine, the next element in seawater in higher concentration is magnesium.
After Magnesium, the next ionic concentration comes in the order of Calcium than Zinc, and then Lead.
Hence, After Na, K, and Cl, seawater has a higher concentration of Mg,
So, the correct answer is “Option A”.

Note:
Seawater is a complex mixture containing various salts, elements, and substances. Chloride, magnesium, sulfate, and calcium, the most abundant dissolved ions in seawater. Ions like calcium tend to precipitate easily. The concentration (mol/kg) of magnesium in seawater is nearly 0.0528.