Molar Mass Of Anhydrous Salt

ii.12: Hydrates

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learning objectives

Define hydrates and identify common hydrates
Outline properties of hydrates
Decide formulas for hydrates

Introduction

H2o, the nigh common chemic on earth, can exist institute in the atmosphere as water vapor. Some chemicals, when exposed to water in the atmosphere, volition reversibly either adsorb it onto their surface or include it in their structure forming a complex in which h2o more often than not bonds with the cation in ionic substances. The h2o present in the latter case is called water of hydration or water of crystallization. Common examples of minerals that exist as hydrates are gypsum (\(\ce{CaSO4*2H2O}\)), Borax (\(\ce{Na3B4O7*10H2O}\)) and Epsom salts (\(\ce{MgSO4*7H2O}\)). Hydrates generally contain h2o in stoichiometric amounts; hydrates' formulae are represented using the formula of the anhydrous (not-water) component of the circuitous followed by a dot and then the water (\(\ce{Water}\)) preceded past a number corresponding to the ratio of \(\ce{H2O}\) moles per mole of the anhydrous component present. They are typically named by stating the name of the anhydrous component followed by the Greek prefix specifying the number of moles of water nowadays so the discussion hydrate (example: \(\ce{MgSO4*7H2O}\): magnesium sulfate heptahydrate).

Backdrop of Hydrates

It is generally possible to remove the water of hydration by heating the hydrate.

The residuum obtained after heating, chosen the anhydrous compound, volition accept a unlike construction and texture and may have a different color than the hydrate.

Example:

\[ \underbrace{\ce{CuSO4*5H2O (s)}}_{\text{Deep Blue}} \ce{->[\Delta]} \underbrace{ \ce{CuSO4 (s)}}_{\text{Ashy White}} \ce{+ v H2o (g)} \label{ane}\]

\[ \underbrace{\ce{CuSO4 (s)}}_{\text{Ashy White}} \ce{->[\ce{H2O (l)} ]} \underbrace{ \ce{CuSO4 (aq)}}_{\text{Deep Blueish}} \label{ii}\]

Whatsoever anhydrous chemical compound from a hydrate generally has the following properties:

Highly soluble in water
When dissolved in water, the anhydrous compound volition take a color similar to that of the original hydrate even if it had changed color going from the hydrate to the anhydrous compound.

Near hydrates are stable at room temperature. All the same, some spontaneously lose water upon standing in the atmosphere, they are said to be efflorescent.

Other compounds can spontaneously absorb water from the surrounding temper, they are said to be hygroscopic. Some hygroscopic substances, such as \(\ce{P2O5}\) and anhydrous \(\ce{CaCl2}\), are widely used to "dry out" liquids and gases (run into experiment on the Molecular Weight of \(\ce{CO2}\)); they are referred to as desiccants. Other hygroscopic substances, such as solid \(\ce{NaOH}\), absorb so much water from the temper that they dissolve in this water, these substances are said to be deliquescent. Some compounds like carbohydrates release h2o upon heating by decomposition of the compound rather than past loss of the water of hydration. These compounds are not considered true hydrates equally the hydration process is not reversible.

Formula of a Hydrate (\(\text{Anhydrous Solid}\ce{*}10\ce{Water}\))

The formula of a hydrate can be adamant by dehydrating a known mass of the hydrate, so comparing the masses of the original hydrate and the resulting anhydrous solid. The mass of h2o evaporated is obtained by subtracting the mass of the anhydrous solid from the mass of the original hydrate (\ref{iii}):

\[m_{\ce{H2o}} = m_{\text{Hydrate}} - m_{\text{Anhydrous Solid}} \label{3}\]

From the masses of the water and anhydrous solid and the tooth mass of the anhydrous solid (the formula of the anhydrous solid will be provided), the number of moles of water and moles of the anhydrous solid are calculated as shown beneath (\ref{4}, \ref{5}):

\[n_{\ce{Water}} = \frac{m_{\ce{H2O}}}{MM_{\ce{Water}}} \label{four}\]

\[n_{\text{Anhydrous Solid}} = \frac{m_{\text{Anhydrous Solid}}}{MM_{\text{Anhydrous Solid}}} \label{five}\]

In guild to determine the formula of the hydrate, [\(\text{Anhydrous Solid}\ce{*}x\ce{H2O}\)], the number of moles of water per mole of anhydrous solid (\(x\)) will be calculated past dividing the number of moles of water past the number of moles of the anhydrous solid (Equation \ref{6}).

\[x = \frac{n_{\ce{Water}}}{n_{\text{Anhydrous Solid}}} \label{6}\]\

Video Tutor:

Practise \(\PageIndex{one}\)

You lot weight out a 0.470-sample of hydrated nickel(2 ) chloride, NiCl₂·tenH₂O. Upon heating, the mass of the anhydrous salt that remains is 0.256 grams. What is the formula of the hydrate? What is the proper name of the hydrate?

Respond

Contributors:

Original Article can be found here: 5: Properties of Hydrates (Experiment)
Modified past Ronia Kattoum (UA of Niggling Stone)