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Which one of the following solutions will have the greatest concentration of hydroxide ions 0.100 M rubidium 0.100 M magnesium hydroxide 0.100 ammonia 0.100 M beryllium hydroxide 0.100 M hydrochloric acid

Amanda Johnson

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Roger Moore on February 18, 2019

Answer : The correct answer is solutions of 0.100 M Magnesium hydroxide and hydroxide of Beryllium.The Hydroxide ions(OH⁻) are produced when ionic compounds (basic compound ) dissociates in water or compound that reacts with water as follows : ministry of health (s) + H2O (l) <=> M+ (aq) + OH⁻(aq) M (aq) + H₂O(l) <=> MH (aq) + OH⁻(aq)Where the ministry of health and M are compounds.The concentration of OH⁻ can be found in the compound by the following steps : Step 1 : find the acid or the base . First of all we need to know is if the given compound is acid or base acid - base concept . (image attached ) If a compound is acid , does not produce OH⁻ ions or negligible amount of OH⁻ ions. So that can be ruled out . If the compound is the base , step 2 should be followed . Step 2 : To find the strength of the base . We need to find that the base is strong or weak .Generally the hydroxides of groups 1 and 2 are a strong base . Here are some of the above-mentioned strong base ( image attached ) .The Basis of these criteria, it is the weak base . Step 3 :To find the concentration of OH⁻ ions: If the base is strong , it means that it dissociates completely to produce OH⁻ ions. Therefore the concentration of OH⁻ can be calculated by using the stoichiometric ratio of the reaction.If the base is weak , do not dissociate completely to give all the OH⁻ ions, so we're going to do ICE table equilibrium of the reaction of the base and use kb values to calculate the concentration of the base . A) hydroxide Rubidium (RbOH ) (0.100 M ) : According to Arrhenius concept RbOH is a basis and the list is a strong base . Therefore, it is to dissociate completely to give OH⁻ ions. RbOH + H₂O <=> Rb⁺ + OH⁻ Since the Ratio of RbOH : OH⁻ = 1 : 1 .[OH⁻] = 0.100 M B) Magnesium hydroxide , Mg(OH)₂ ( 0.100 M) : According to the concept of Arrhenius, and since Mg is in group 2 , so that the Mg(OH)₂ can be said as a Solid foundation . Thus, Mg(OH)₂ fully dissociate as : Mg(OH)₂ + H₂O <=> Mg2⁺ + 2 OH⁻ The ratio of OH⁻ : Mg(OH)₂ = 2 : 1[OH⁻ ] = 0.200 M C) Ammonia (NH3 ) (0.100 M ) : According to Lewis concept it is base, but it does not belong to the list or the group 1 and group 2 , therefore it is a weak base and will dissociate completely as : NH₃ + H₂O <=> NH₄OH <=> NH₄⁺ + OH⁻, Because it is a weak base, so that the ICE table is formed as: NH₃ + H₂O <=> NH₄⁺ + OH⁻ Initial(M) 0.100 , - 0 0Change(M) -x +x + xEquilibrium(M) 0.100-x - x xNow Kb expression will be written as : Kb for NH3 is 1.8 * 10 ⁻⁵ ( from the list in the image ) Plugging the equilibrium values of [NH₄⁺] = [OH⁻] = x and [NH₃] = (0.100-x ) in this expression : Since kb is small (10⁻⁵ ) , x (0.100 -x ) can be neglected Multiplying both sides by 0.100 x2 = 1.8 * 10⁻⁶ x = 1.34 * 10-3 therefore the concentration of OH⁻ produced by NH₃ = 1.34 * 10-3 M D) hydroxide of Beryllium ,Be(OH)₂ (0.100 M ) :According to Arrhenius concept of what a base is and, as it belongs to Group 2 , is a strong base . Therefore, it is to dissociate completely . Be(OH)₂ + H₂O <=> Be2⁺ + 2 OH⁻ Relationship between the OH⁻ : Be(OH)₂ = 2 : 1 therefore [OH⁻] Be(OH)₂ = 0.200 M E) Hydrochloric acid , HCl (0.100 M ) :Agree with all the concepts HCl is an acid . Since that is the acid produced negligible amount of OH⁻ ions. Therefore, this option can be neglected . If everything is summarized : [OH⁻] by RbOH = 0.100 M [OH⁻] for the Mg(OH)₂ = 0.200 M [OH⁻] by NH₃ = 1.34 * 10-3 M[OH⁻] Be(OH)₂ = 0.200 M [OH⁻] HCl = negligible, therefore, the Mg(OH)₂ and ba(OH)2 produces more amount of [OH⁻] .


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